Vacuum Pyrolysis of Hybrid Poplar Milled Wood Lignin with Fourier Transform-Ion Cyclotron Resonance Mass Spectrometry Analysis of Feedstock and Products for the Elucidation of Reaction Mechanisms

Abstract

The pyrolysis of lignocellulosic materials is a promising technique to produce fuels and chemicals. It is well known that the most abundant products of lignin pyrolysis are oligomeric molecules, known as pyrolytic lignin (PL). The chemical composition of PL has been extensively studied; however, there is still an important debate whether these oligomers are produced directly from the lignin or from the recombination of monomeric pyrolytic products. Existing theories are unable to describe the effect of vacuum on the distribution of pyrolysis products. Hybrid poplar milled wood lignin (MWL) was initially isolated and thoroughly characterized by Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS). Chemical formulas were assigned to each oligomeric compound detected. The MWL was also subjected to vacuum pyrolysis in a modified pyroprobe at 250, 750, and 1000 mbar (absolute pressure), and the resulting liquid products were analyzed by FT-ICR MS. A new strategy to assign structural representations to the oligomeric PL products is proposed, based on the plausible pyrolysis reaction mechanisms of depolymerization/fragmentation applied to original MWL oligomer formulas. Our results support the hypothesis that PL is formed from the removal of moieties from primary lignin pyrolysis products with between three and five aromatic rings. This depolymerization/fragmentation allows the oligomers to reduce their molecular weights to the point where they can be removed from the reaction zone by direct vaporization. This phenomenon highlights the importance of pressure on removal mechanisms and their impact on the molecular weight of the resulting products from lignin pyrolysis.

Thermodynamic and Physical Property Estimation of Compounds Derived from the Fast Pyrolysis of Lignocellulosic Materials

“…The structural formula of the LMM pyrolytic lignin molecule has been proposed in this work and is also shown in section S.1 of the Supporting Information. The molecular representation proposed by Terrell and Garcia-Perez [G­(βO4)­G­(βO4)­G–H 2 ] and the possible appearance of quinone groups during pyrolysis were taken into account to define a plausible LMM pyrolytic lignin structural formula. This molecular representation [G­(βO4)­G­(βO4)­G–H 2 ] is based on three guaiacol monomer units (typical in softwood lignin) connected with β–O–4 bonds, which are the most abundant bonds in lignin.…”

“…The structural formula of the LMM pyrolytic lignin molecule has been proposed in this work and is also shown in section S.1 of the Supporting Information. The molecular representation proposed by Terrell and Garcia-Perez 35…”

Secondary Reactions of Primary Tar from Biomass Pyrolysis: Characterization of Heavy Products by FT-ICR MS

“…The mass distribution of primary tar centered at nearby m / z 300. Most of peaks in mass spectra should be assigned to lignin derivatives as oligomers . These oligomeric compounds can be roughly divided into lignin dimers, trimers, and tetramers on the basis of their mass groups .…”

“…Most of peaks in mass spectra should be assigned to lignin derivatives as oligomers. 20 These oligomeric compounds can be roughly divided into lignin dimers, trimers, and tetramers on the basis of their mass groups. 30 Remarkably, dimers (m/z 250−350) were dominant in primary tar (Figure 2a), while the relative intensity of other products is obviously lower than that of dimers.…”

“…Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICRMS) is able to discriminate the thousands of peaks of the mass spectra and to assign each m / z by their unique elemental formula (i.e., C x H y O z ) . Hence, FT-ICRMS has been successfully applied for the direct characterization of bio-oil components and also for interpreting their reaction processes. − Different types of ionization sources like electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), laser desorption/ionization (LDI), and atmospheric pressure photoionization (APPI) could be equipped with FT-ICRMS on the basis of the character of the target compounds to be analyzed. Among them, APPI has been considered as an efficient method to ionize both nonpolar and polar species, especially for polycyclic aromatic hydrocarbons (PAHs). − Moreover, it has been evidenced that the APPI method performs better in the ionization of large molecules with higher double-bond equivalent (DBE) values and lower polarity than other ionization sources .…”