Activation of bio-oil with or without pre-carbonization makes marked difference in pore development

Abstract

Bio-oil, a major product from biomass pyrolysis, is a renewable source for production of functional carbon materials such as activated carbon (AC). Bio-oil can be activated directly via mixing with an activator or be pre-carbonized followed by subsequent activation. These two processes feature with distinct reaction network and might affect pore characteristics in different ways. This was investigated herein by direct activation of bio-oil or biochar from pre-carbonization of bio-oil using K₂C₂O₄ or KOH as the activator at 800 ºC. The results indicated that bio-oil was pre-carbonized into biochar at 500 ºC improved yields of AC (83.6 % versus 17.5 % from direct activation of bio-oil with K₂C₂O₄) through enhanced aromatic degree and resistivity to cracking. This also diminished specific surface area of resulting AC (716.3 versus 1035.5 m²g⁻¹ from direct activation). Much more intensive cracking reactions in direct activation of bio-oil with K₂C₂O₄ were confirmed with in-situ IR technique. This not only formed AC of more developed pore structures, especially more mesopores/macropores, but also generated gases as dominate product (yield: 74.5 %). Similar result was observed from activation with KOH, but KOH was more effective than K₂C₂O₄ for cracking. This, however, did not result in AC of more developed pore structures, as the K₂CO₃ derived from KOH tended to be wrapped with organics in direct activation of bio-oil and was difficult to be washed away, blocking pores generated.