Abstract
The transformation of lignocellulosic biomass into bio‐based commodity chemicals is technically possible. Among thermochemical processes, fast pyrolysis, a relatively mature technology that has now reached a commercial level, produces a high yield of an organic‐rich liquid stream. Despite recent efforts to elucidate the degradation paths of biomass during pyrolysis, the selectivity and recovery rates of bio‐compounds remain low. In an attempt to clarify the general degradation scheme of biomass fast pyrolysis and provide a quantitative insight, the use of fast pyrolysis microreactors is combined with spectroscopic techniques (i.e., mass spectrometry and NMR spectroscopy) and mixtures of unlabeled and 13C‐enriched materials. The first stage of the work aimed to select the type of reactor to use to ensure control of the pyrolysis regime. A comparison of the chemical fragmentation patterns of “primary” fast pyrolysis volatiles detected by using GC‐MS between two small‐scale microreactors showed the inevitable occurrence of secondary reactions. In the second stage, liquid fractions that are also made of primary fast pyrolysis condensates were analyzed by using quantitative liquid‐state 13C NMR spectroscopy to provide a quantitative distribution of functional groups. The compilation of these results into a map that displays the distribution of functional groups according to the individual and main constituents of biomass (i.e., hemicelluloses, cellulose and lignin) confirmed the origin of individual chemicals within the fast pyrolysis liquids.
Highlights
The fast pyrolysis of plant biomass has reached the technological and commercial maturity to convert solid materials into bio-oil.[1]
Cellulosic and hemicellulosic fractions were obtained using the classical method of a two-step sulfur-free soda pulping with sodium boron hydride in the first step to protect soluble hemicelluloses[22] followed by further purification through selective bleaching and extraction steps to separate cellulose from hemicelluloses
If we consider the yields of individual key products for biooils derived from mixtures, the production of cellulose-derived products was not enhanced and that of lignin-derived products was substantially suppressed. These results indicate that the reported beneficial effect of the presence of lignin on cellulose degradation and vice versa during primary pyrolysis[35] was inhibited by the presence of hemicelluloses. These results indicate that the mechanistic explanation suggested by Hosoya et al.[35] that the “polymerization of anhydrosugars is inhibited by the lignin-derived volatile products” to the benefit of oxygenated fivecarbon heterocycles production is unlikely to happen under these fast pyrolysis conditions
Summary
Most of the degradation pathways that have been suggested to date are based on the thermal degradation of model compounds, which leads to oversimplified degradation schemes and a biased picture of the composition of bio-oil These studies have been instrumental to reveal key patterns. Despite significant progress over the last 30 years, a fundamental understanding of fast pyrolysis chemistry, that is, the key mechanistic details that lead to the formation of fast pyrolysis bio-oil, is still lacking for a number of reasons: (i) the identification of chemical reactions based on the conversion of model compounds often leads to oversimplified degradation schemes; (ii) the inability of analytical techniques to describe bio-oil immediately, fully and unequivocally; and (iii) the control of the pyrolysis regime is often impractical. This study confirms and clarifies the general degradation scheme for biomass fast pyrolysis by providing a quantitative insight
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