Molecular-Level Understanding of the Major Fragmentation Mechanisms of Cellulose Fast Pyrolysis: An Experimental Approach Based on Isotopically Labeled Model Compounds

Abstract

Evaluation of the feasibility of various mechanisms possibly involved in cellulose fast pyrolysis is challenging. Therefore, selectively 13C-labeled cellotriose, 18O-labeled cellobiose, and 13C- and 18O-doubly-labeled cellobiose were synthesized and subjected to fast pyrolysis in an atmospheric pressure chemical ionization source of a linear quadrupole ion trap/orbitrap mass spectrometer. The initial products were immediately quenched, ionized using ammonium cations, and subsequently analyzed using the mass spectrometer. The loss or retention of isotope labels upon pyrolysis unambiguously revealed three major competing mechanismssequential losses of glycolaldehyde/ethenediol molecules from the reducing end (the reducing-end unraveling mechanism), hydroxymethylene-assisted glycosidic bond cleavage (HAGBC mechanism), and Maccoll elimination. Important discoveries include the following: (1) Reducing-end unraveling is the predominant mechanism occurring at the reducing end; (2) Maccoll elimination facilitates the cleaving of aglyconic bonds, and it is the mechanism leading to formation of reducing carbohydrates; 3) HAGBC occurs for glycosides but not at the reducing end of cellodextrins; 4) HAGBC and water loss are the predominant reactions for fast pyrolysis of 1,6-anhydrocellodextrins; and 5) HAGBC can proceed after reducing-end unraveling but unraveling does not occur once the HAGBC reaction pathway is initiated. Moreover, hydrolysis was conclusively ruled out for fast pyrolysis of cellobiose, cellotriose, and 1,6-anhydrocellodextrins up to cellotetraosan. No radical reactions were observed.

Recent Progress in Quantum Chemistry Modeling on the Pyrolysis Mechanisms of Lignocellulosic Biomass

“…This process involves a hydride migration between a hydroxyl group and its meta -carbonyl group, accompanied by the scission of a C–C connection and the formation of unsaturated CO and CC bonds. The retro-aldol reaction has been reported responsible for the fragmentation of the glucose units with relatively low energy barriers. ,,, Figure b shows the most typical retro-aldol reaction. Because the hydroxyl group is a typical structural characteristic of the glucose unit, the presence and site of the carbonyl group determine the feature of the retro-aldol reaction.…”

“…The biomass pyrolysis mechanisms, especially formation channels of the pyrolytic products, have been a long-lasting focus for the research community. Advanced experimental approaches have been employed to obtain important results, such as thermogravimetric analysis coupled to Fourier transform infrared spectrometry (TG–FTIR), in situ diffuse reflectance infrared Fourier transform spectroscopy ( in situ DRIFT), − pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS), , pyrolysis–imaging photoelectron photoion coincidence spectroscopy (Py–iPEPICO), synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV–PIMS), ,, isotopic labelings, , etc. Besides, computational methods are also powerful tools for the mechanism study of biomass pyrolysis.…”

Exploring the Reaction Mechanisms of Fast Pyrolysis of Xylan Model Compounds via Tandem Mass Spectrometry and Quantum Chemical Calculations

“…This is important in order to better understand the reaction mechanisms involved in the initial depolymerization steps during hemicellulose/xylan fast pyrolysis. 14,15,36,37 9) was purchased from Indiana Oxygen. All chemicals were used without further purification.…”

“…Furthermore, much larger compounds, including oligomers, can be detected using this setup than by using GC/MS. This is important in order to better understand the reaction mechanisms involved in the initial depolymerization steps during hemicellulose/xylan fast pyrolysis. ,,, Quantum chemical calculations were utilized to explore possible reaction mechanisms for formation of these products.…”

Glycosidic C–O Bond Activation in Cellulose Pyrolysis: Alpha Versus Beta and Condensed Phase Hydroxyl-Catalytic Scission

“…This is in line with other studies that have found no evidence for hydrolysis to be a primary pathway. 25,26 These differences between theory and experiment suggest a need for re-examining the intramolecular bondmaking and bond-breaking events that influence glycosidic bond activation with a special emphasis on explaining the peculiar similarity between α and β-isomers.…”