Advances in lignin chemistry during pulping and bleaching

Abstract

Lignin, a key component of plant cell walls, undergoes significant structural changes during pulping and bleaching processes that critically impact pulp quality and paper properties. This review systematically examines recent advances in elucidating the structural and chemical transformations of lignin during pulping and bleaching processes, with a focus on reaction mechanisms and their industrial implications. In acidic pulping, protonation and hydrolysis of ether bonds lead to lignin depolymerization, while competing condensation reactions form carbon-carbon linkages. Alkaline pulping primarily cleaves β-O-4 bonds using quinone methide intermediates, which fragment lignin and enable condensation. Different reagents exhibit distinct mechanisms during bleaching: Chlorine dioxide phenolic groups form quinones and cleave aromatic rings, hydrogen peroxide-generated hydroxyl radicals preferentially attack non-phenolic structures, and ozone directly disrupts electron-rich aromatic regions through cycloaddition. These chemical changes not only facilitate lignin removal but also affect pulp’s brightness, strength and environmental impacts. This review provides a fundamental framework of lignin chemistry during these processes for developing more efficient, sustainable pulping technologies.