Beyond pretreatment: Oriented valorization of biomass into platform chemicals via torrefaction mediated by acid-base balance

Abstract

Torrefaction has been usually considered as an efficient pretreatment method for improving the fuel properties of biomass, such as heating value, hydrophobicity, and grindability. Herein, it is found that oriented torrefaction mediated by acid-base balance is an efficient method capable of achieving selective valorization of biomass into levoglucosan. The effects of different types of exogenous acids and their loading amounts on the torrefaction behaviors of pine at 300 °C were systematically investigated. It is demonstrated that 0.4% H3PO4 and 0.4% H2SO4 loading can sharply improve the yield of levoglucosan from 3.5 to their respective maximum values of 52.9 and 63.6% during the torrefaction of pine at 300 °C, whereas HCOOH, CH3CHOOH, HCl, and HNO3 loading cannot significantly improve the yield of levoglucosan, indicating that the anions of exogenous acids are of great importance for achieving oriented torrefaction. The optimal loading amount of H2SO4 or H3PO4 for achieving oriented torrefaction of biomass is equal to the inherent alkali and alkaline earth metals (AAEM) content of biomass divided by its cellulose content. It is inferred that the exogenous acid-inherent AAEM balance is the key to achieving oriented torrefaction of biomass into levoglucosan. Exogenous H2SO4/H3PO4 could serve as acidic catalysts for achieving low-temperature activation of cellulose into levoglucosan, and inherent AAEM could act as basic inhibitors for suppressing dehydration reactions to form levoglucosenone. The pyrolysis kinetic analysis shows that 0.4% H3PO4 loading can promote the activation energy for the pyrolysis of pine. These findings provide theoretical guidelines for further understanding and developing the oriented torrefaction process of biomass mediated by acid-base balance.