On the formation of diphenylmethane structures in lignin under kraft, EMCC® and soda pulping conditions

This paper describes our efforts focused at defining the formation of diphenylmethane moieties in lignin as they form during conventional and EMCC(R) kraft and soda pulping conditions. This objective was realized by initially confirming the assignment of a P-31 NMR signal as being exclusively due to the presence of phosphitylated diphenylmethane (DPM) phenolic hydroxyl groups. More specifically, softwood milled wood lignin (picea mariana) was subjected to kraft pulping conditions in the presence and absence of varying amounts of formaldehyde. After quantitative recovery of the lignin, the P-31 NMR spectra were recorded and the spectra revealed selective signal growth in the region confined between 142.8-144.3 ppm, in accordance with previous model compound work and detailed calculations based on the Hammett principles. To further substantiate our conclusions we also carried out two series of isothermal (120 degrees C) kraft and soda pulping experiments followed by quantitative determinations of the DPM moieties and correlated our findings with the differences in chemistry known to occur between the two processes. Finally, diphenylmethane phenolic moieties were determined in isolated residual lignins from two western hemlock kraft pulp samples produced via an EMCC(R) mill protocol and a laboratory batch digester, respectively. These structures were found to prevail amongst the condensed phenolic units by 33% in the conventional cook, providing additional evidence supporting the fact that modern modified cooking technologies alter the structure of residual kraft lignin beneficially.