Bio-Based Phenol from Cashew Nutshells by Catalytic Hydrocardanol Trans-Alkylation Using H-ZSM-5 Zeolite.

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Cardanol, an unsaturated long-chain alkylphenol derived from agricultural waste, has potential as a sustainable substrate for bio-based phenol production. Herein, we demonstrate the potential of hydrocardanol, readily obtained through cardanol hydrogenation, as a viable feedstock for phenol production via trans-alkylation using toluene as the alkyl acceptor. HZSM-5 (SiO2/Al2O3 molar ratio = 80) exhibited a 53.7% phenol yield with a high phenolics balance (86.7%) from a 10 wt% hydrocardanol solution at full conversion in a batch reactor. In contrast, cardanol gave only 27.1% phenol under the same reaction conditions. This improved yield with hydrocardanol is attributed to suppressed formation of bi- and/or polycyclic phenols via self-alkylation, due to the higher thermal stability of the saturated side chain. Re-alkylation of the desired phenol product largely limited the phenol yield. The model reaction experiments with phenol and 1-pentadecene showed that the reaction proceeds via hydrocardanol dealkylation on the zeolite, followed by rapid isomerization, cracking and oligomerization reactions of the resulting long-chain olefin, and re-alkylation of toluene and phenol. This indicated that phenol alkylation could be suppressed by reducing the hydrocardanol-to-toluene ratio. This approach was validated in a fixed-bed flow reactor, achieving a high phenol yield (>95%) with a high carbon balance (>99%).