Microwave-Assisted Ionic Liquid Pretreatment and Catalysis of Lignin to Phenol in a Biphasic Reaction System

Microwave-assisted Ionic Liquid Pretreatment and Catalysis of Lignin to Phenol in a Biphasic Reaction System

Yongshui Qu, Hao Luo, Jian Xu*

National Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, People's Republic of China

*Corresponding author: jxu@home.ipe.ac.cn

Abstract: Recent concerns about the gradual depletion of conventional fossil resources and the pressure from global climate change have accentuated the need for new alternative resources. As one of the main components in biomass, lignin is the second most abundant natural polymer after cellulose which has the potential to serve as a sustainable source of energy and organic carbon to replace petroleum-based chemicals. Efficient conversion of lignin into high value-added chemicals is a key to improve the economic feasibility of biomass refinery. In the present study, a biphasic reaction system assisted by microwave heating is employed in the conversion of lignin into phenol, a vital petroleum-based building blocks used in production of fine chemicals and plastics. In order to understand the relationship among the structure of the ionic liquid, its effect on the change of lignin structure and yield of phenol, a series of functional ionic liquids are prepared to pretreat the industrial lignin and the changes on the structures and characteristics of the lignin (functional groups, microcosmic morphology, etc.) in the pretreatment and its effect on the phenol production are investigated. The study on the directional conversion of the pretreated lignin to phenols in a biphasic system is subsequently carried out by using the microwave irradiation instead of conventional heating, replacing the common catalysts (acids or alkalis) with the ionic liquids. The result shows that the phenol yield of 8.14% is achieved with the microwave irradiation at 400 W for 60 min and the produced phenol can be separated easily by a separation-friendly biphasic system. In this biphasic system, the industrial lignin pretreated by 1-ethyl-3-methylimimidazolium acetate ([EMIM]OAc) is catalyzed into phenol by an alkaline ionic liquid. The phenol product is continuously extracted into an organic phase consisting of methylisobutylketone (MIBK) and 2-butanol.

Keywords: Lignin; Ionic liquid; Pretreatment; Phenol; Catalysis