Abstract
Aldehydes and ketones are known to play a role in the aging process of pyrolysis oil and generally, aldehydes are known for their high reactivity. In order to discern in pyrolysis oil the total aldehyde concentration from that of the ketones, a procedure for the quantification of aldehydes by 1H-NMR was developed. Its capability is demonstrated with a hardwood pyrolysis oil at different stages of the aging process. It was treated by the Accelerated Aging Test at 80 °C for durations of up to 48 h. The aldehyde concentration was complemented by the total concentration of carbonyls, quantified by carbonyl titration. The measurements show, that the examined hardwood pyrolysis oil contained 0.31–0.40 mmol g−1 aldehydes and 4.36–4.45 mmol g−1 ketones. During the first 24 h, the aldehyde concentration declined by 23–39% and the ketone concentration by 9%. The rate of decline of aldehyde concentration slows down within 24 h but is still measureable. In contrast, the total carbonyl content does not change significantly after an initial decline within the first 4 h. Changes for vinylic, acetalic, phenolic and hydroxyl protons and for protons in the α-position to hydroxy, ether, acetalic and ester groups were detected, by 1H-NMR. In the context of characterizing pyrolysis oil and monitoring the aging process, 1H-NMR is a reliable tool to assess the total concentration of aldehydes. It confirms the reactivity of aldehydes and ketones and indicates their contribution to the instability of pyrolysis oil.
Highlights
The pyrolysis oil and its aging process can be described by the aldehyde and ketone concentration, which can be seen as key-indicators in this context
Esteri cation is strongly suggested in Fourier-transform infrared spectroscopy (FTIR) spectra of pyrolysis oil[40,43,44,74] by absorption bands at 1700 cmÀ1 and 1200 cmÀ1.59 Absorption around 1200 cmÀ1 can be assigned to many other compound classes like ethers (n(C– O)), acids and especially sulfates (n(S]O); n3 mode of SO4 group), but signals around 1700 cmÀ1 usually belong to carbonyls, and coupled with the absorption band at 1200 cmÀ1 to esters (asymmetric and symmetric stretching modes of the carboxylic acid, nas(COOÀ) and ns(COOÀ) resp.).[59]
Before aging the aldehyde content amounts to a concentration of 0.40 mmol gÀ1, when deduced from 1H-nuclear magnetic resonance spectroscopy (NMR) spectra recorded at 400 MHz, whereas spectra at 44 MHz delivered a concentration of 0.31 mmol gÀ1
Summary
Biomass has been extracted for diverse purposes. Besides nutritional reasons, it has always served as a material in the construction sector[1] for instance and as a common direct source of energy.[2]. The pyrolysis oil and its aging process can be described by the aldehyde and ketone concentration, which can be seen as key-indicators in this context.
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