Alkaline hydrothermal conversion of cellulose to bio-oil: Influence of alkalinity on reaction pathway change

The effects of alkalinity on alkaline hydrothermal conversion (alkaline-HTC) of cellulose to bio-oil were investigated in this study. The results showed that the initial alkalinity greatly influenced the reaction pathways. Under initial strong alkaline conditions with final pH greater than 7, alkaline-HTC only followed the alkaline pathway. However, under initial weak alkaline conditions with final pH of less than 7, acidic as well as alkaline pathways were involved. The main mechanism behind this change of reaction pathways under weak alkaline conditions was that carboxylic acids were first formed from cellulose via the alkaline pathway and then neutralized/acidified the alkaline solutions. Once the pH of the alkaline solutions decreased to less than 7, the acidic instead of the alkaline reaction pathway occurred. This change of the reaction pathways with initial alkalinity partly explained the inconsistent results in the literature of alkaline-HTC bio-oil compositions and yields.