Experimental techniques to gain mechanistic insight into fast pyrolysis of lignocellulosic biomass: A state-of-the-art review

This review focuses on identifying recent advances in experimental techniques used to probe gas-phase reactive/ stable intermediates in fast pyrolysis (FP). Potential spectroscopic tools are discussed that can be employed to gain molecular level understanding of the surface adspecies in catalytic fast pyrolysis (CFP) of lignocellulosic biomass. Isotopic labeling studies coupled with advanced analytical systems, such as dopant-assisted atmospheric pressure chemical ionization time-of-flight mass spectroscopy (dAPCI-TOFMS) and synchrotron-based vacuum ultraviolet photoionization mass spectroscopy (SVUV-PIMS), are specifically noted. These methods have rejected certain reaction pathways proposed by density functional theory (DFT) based studies, which has promoted new alternative mechanisms to be proposed. The discussion includes similarities in the hydrocarbon pool species (HCP) formed during methanol-to-olefins (MTO) process and CFP of biomass along with the spectroscopic studies used to characterize the HCP species in MTO process on zeolite-based catalysts. While numerous reviews regarding pyrolysis reactors, intrinsic reaction mechanisms, and advanced analytical techniques to characterize bio-oil components are available in the open literature, the primary focus of this review is to identify the experimental studies that provide molecular-level mechanistic information on FP and CFP of biomass. Finally, future perspectives and key challenges are discussed.