摘要
Phosphoric acid is an effective and widely used catalyst for biomass pyrolysis, with significant catalytic impact on the pyrolytic reactions and product distribution. However, its fundamental catalytic mechanism in lignin pyrolysis remains unclear. In this work, we proposed and investigated three possible H 3 PO 4 -assisted pyrolysis mechanisms, i.e., hydrogen-bond-induced decomposition, phosphoric-acid-assisted hydrogen transfer (H 3 PO 4 -AHT), and phosphorolysis, and carefully discussed them through theoretical calculations, electronic structure analyses, catalytic pyrolysis experiments using the β -O-4 linked lignin model. The results indicated that the H 3 PO 4 -AHT mechanism played a dominant role in lignin catalytic pyrolysis, superior to the other two mechanisms. The phosphoric acid molecule could take advantage of its unsaturated P = O and saturated P −OH groups for hydrogen donation and acceptance in concerted H 3 PO 4 -AHT reactions. Alternatively, it could also participate in concerted intermolecular hydrogen transfer with the active hydroxyl group as a mediator. The C β −O bond cleavage could be significantly enhanced by effectively reducing the tension of the transition state ring, leading to the generation of more phenol and styrene in pyrolysis experiments. Due to the inhibition of hydrogen abstraction, C α −C β breakage of β -O-4 linkage, side chain removal of primary products, etc., the formation of other products in phosphoric acid catalytic pyrolysis of β -O-4 linked lignin was restrained. This H 3 PO 4 -AHT catalytic mechanism was also confirmed to be conductive to tautomerism, elimination, dehydration, α -O-4 linkage fracture, β -1 linkage fracture, and other important decomposition reactions in lignin pyrolysis. The understanding of these mechanisms provides valuable insights into the role of phosphoric acid in lignin pyrolysis and may guide the development of effective biomass catalytic conversion methods.
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