Substituent Effects in the Pyrolysis of Lignin Model Compounds

A systematic study of the impact of PPE substituents on the thermal decomposition of lignin model compounds was performed. Lignin is the second most abundant natural biopolymer found in vascular plants and a potential source for renewable chemicals and fuel. The pyrolysis of the o′-OCH₃-PPE (phenethyl phenyl ether) produced products consistent with the proposed free radical pathway described for PPE. The major products were styrene and guaiscol in a 1:1 ratio and < 10 mole each of toluene, ethyl benzene, anisole, phenol, and o-anisaldehyde. The o′- methoxy substituent reduced the rate of the rate of the 1,2-phenyl shift. The major products were styrene and 4-methoxyphenol produced in a 1:1 ratio, analogous to o′-OCH₃-PPE. The o′-methoxy substituent imposed steric constraints on the reactions. The methoxy-substituent also enhanced hydrogen abstraction at the β-carbon. Acetophenone and phenol were the dominant products formed (25.5 and 39.6 mole %, respectively) in the thermolysis of α-OH-PPE. Benzene, toluene, styrene, benzaldehyde, phenol, benzyl alcohol, phenylacetaldehyde, and dehydrated α-OH-PPE were formed in smaller amounts (0.80-7.0 mole %). Phenol and p-vinylphenol made up the largest fraction of the product mixture 41.4 and 37.2 mole%, respectively, from the hydrogen abstraction at the α-carbon. The rate of reaction increased if the substituent could stabilized the radicals formed from the C-O homolysis or if the substituent increased the rate constant for hydrogen abstraction. A subtle balance between rates for competitive hydrogen abstraction 1,2-phenyl shift and β-scission steps controlled the product selectivities.