To provide essential understanding for lignin degradation mechanisms, we have focused on three main approaches: 1) mass balance closure using a newly developed thermal carbon analysis (TCA), 2) evaluation of main structural features through phosphitylation of OH-containing groups and 31P nuclear magnetic resonance (NMR) spectroscopy, and 3) molecular weight (MW) determination employing gel permeation chromatography with mass spectrometry (GPC-MS). TCA is based on carbon fractionation with increasing temperature (100-850 °C) without and with oxygen, respectively. This method allows to differentiate monomeric species evolving at low temperature steps (200–300 °C) from large MW compounds pyrolyzed at 400–850°C, and finally the “coke” fraction, which evolves only in presence of oxygen at high temperatures. The “coke” carbon cannot be observed with Py-GC-MS, as no oxygen is used. Our results support the need of using TCA for quantification demonstrating that the “coke” carbon represents a significant portion of all characterized samples including both raw and hydrotreated lignin. Moreover, the phenolic-oligomeric fraction is not amenable to GC. Yet, as we demonstrated, this potentially valuable mixture of nano-sized phenolic oligomers can be quantified using TCA with a complementary Py-GC-MS speciation. Phosphitylation followed by 31P NMR spectroscopic analysis allows for identification and quantification of the majority of OH-containing compounds formed during lignin hydrotreatment. Particularly, this method readily identifies phenols, alcohols and carboxylic acids. Comparison of 31P NMR spectra of lignin before and after a hydrotreatment may also provide useful information on possible repolymerization of degradation products, i.e., structure of nano-size oligomers. The application of GPC is limited due to lignin and its oligomers’ heteropolymeric nature and lack of representative standards. Thus, standards were synthesized and a protocol for their analysis was optimized. We confirmed that GPC methods are affected by the superposition of the molecular sieve effect and unwanted analyte-column interactions. The study was carried out using a time of flight MS equipped with electrospray ionization (ESI). We have demonstrated advantages of detection with ESI-high resolution MS providing accurate masses for high MW species of the oligomeric fraction, however, detailed optimization is essential to prevent excessive fragmentation and loss of molecular ions.

Essential analytical approaches to characterization of lignin specifically targeting the abundant oligomeric fraction observed following a hydrotreatment

253rd National Meeting (2017)

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