The catalytic effects of anthraquinone (AQ) and 2-methylanthraquinone (MAQ) significantly improve the efficacy of soda pulping, so much so that soda-AQ (SAQ) and soda-MAQ (SMAQ) hardwood pulps are nearly equivalent to kraft pulps in overall quality. One minor deficiency is a lower bleached brightness compared to kraft pulps particularly when oxygen bleaching is utilized. Some prior pulping and bleaching results suggested that the residual lignin (RL) in the fibers of SAQ and SMAQ pulps were more condensed (C-C bonds) than the lignin in kraft pulps. In the present investigation, it was determined by permanganate oxidation analyses that the RL in SAQ and SMAQ pulps contained ∼3 more C-5 condensed phenylpropane units than the kraft RL. These C-5 condensed structures are known to be less reactive to oxygen and possibly other bleaching chemicals.

A detailed investigation with lignin model compounds (LMCs) showed that C-5 carbanions condensed with o- and p-quinone methides and that the dominant condensation mechanism involved C-5 carbanions reacting with the α-carbon in p-quinone methide. It is well established that one of the main differences between the kraft and SAQ pulping mechanisms is that the main depolymerization reaction (cleavage of β-O-4 bonds) generates a much higher yield of coniferyl alcohol (CA) in the case of SAQ. Some of the CA is transformed to vinylguaiacol (VG) and isoeugenol (IE). It was shown that VG could link two lignin oligomers by condensing with one at its á-carbon position and with the other at its C-5 position. When ethylguaiacol (EG) was added to SAQ cooking of wood meal from sugar maple (Acer saccharum) it formed dimers with CA, VG and IE corresponding to 93% of the estimated theoretical yield of CA. Those dimers had free C-5 positions but did not take part in further condensation to any significant degree. When EG was added to SAQ pulping of sugar maple chips a pulp with equal or superior bleachability to the corresponding kraft pulp was obtained at a higher fiber yield.

Key Words: Anthraquinone, Alkaline pulping, lignin condensation, quinone methide, coniferyl alcohol, carbanion.