3.4. Analysis of cellulosic biomass by 13C CP MAS ssNMR
Using the pristine lignin extracted from kenaf, we measured the
lignin content in kenaf, corn stover and cotton stalks. These lignocellulosic
biomasses are sources for papermaking, biofuel and bioethanol
industries. Kenaf showed the highest lignin content based
on 13C CP/MAS ssNMR and along the other biomass samples gave
lignin values consistent with those reported in the literature (Supporting
Information Table S3).
The 13C CP MAS ssNMR of kenaf on which fungi were cultured
revealed the lignin content was quite different for G. lucidum
(white rot) and F. pinicola (brown rot) (Fig. 6, Table 2). The white
rot G. lucidum fungi digested over 16% kenaf lignin reducing it to
26%, while as expected; the brown rot F. pinicola fungi did not alter
the kenaf lignin.
Quantitative 13C NMR has been applied to lignin qualitative and
quantitative studies since the 1980s [29]. However, application of
high-resolution 13C spectra has been limited due to low natural
abundance of the 13C isotope, difficulties in lignin extraction, and
high lignin concentrations required to achieve good signal-to-noise
(S/N) ratios. The development of inverse detection in NMR (i.e., 2-D