Isolation and structural characterization of the milled-wood lignin from Paulownia fortunei wood

The chemical structure of the milled-wood lignin isolated from Paulownia fortunei wood was investigated. The lignins were characterized by analytical pyrolysis and two-dimensional NMR that indicated a predominance of guaiacyl (G) over syringyl (S) units, and only showed small amounts of p-hydroxyphenyl (H) units, with a H:G:S molar ratio of 1:59:40. The heteronuclear single quantum correlation (HSQC) NMR spectrum gave additional information about the relative abundances of the different inter-unit linkages present in the lignin structure. Paulownia lignin showed a predominance of β-O-4′ aryl ether linkages (62% of total side-chains), followed by β-β′ resinol-type (12%) and β-5′ phenylcoumaran-type structures (11%) and with lower amounts of other condensed structures such as spirodienone (3%) and dibenzodioxocin (2%) structures. The high lignin content (ca. 23% Klason lignin), together with the low S/G ratio and the abundance of condensed (carbon–carbon linked) structures (together with the low abundance of the most labile ether-linked structures) could contribute to the low reactivity of paulownia lignin during alkaline pulping.