Pretreatment of lignin by electrochemical hydrogenation to enhance the olefins and aromatic products during rapid pyrolysis

Abstract

Lignin is one of the most promising biopolymers for producing hydrocarbons and aromatic compounds in nature. Electrochemical conversion of lignin is regarded as an attractive technology for producing various aromatic chemicals which are generally derived from fossil fuels. The author reports an electrochemical approach for hydrogenating alkaline lignin (AL), which uses relatively low-cost Ni foam as a working electrode under alkali electrolyte. The primary factors (such as power input, reactor type, temperature, reaction time, catalysts, and electrode substrates) of electrochemical hydrogenation were used to control the products' yield and features. The redox characteristics of the reaction system were directly compared with the cyclic voltammetry measurement under different parameters. After electrochemical hydrogenation (ECH), rapid pyrolysis was conducted on HAL samples to investigate the influence of electrochemical reactions on lignin. The maximum olefins and aromatics yield from HAL was increased to more than three times AL, accounting for 2.22 wt% and 4.07 wt%, respectively. Then, thermogravimetric analysis (TGA), elemental analysis (EA), Fourier-transform infrared spectroscopy (FTIR), and heteronuclear single quantum coherence (HSQC) NMR were employed to characterize hydrogenated lignin samples (HAL), as well as gas chromatography–mass spectrometry (GC-MS) was conducted to investigate the oil-phase products. This work focused on this electrochemical hydrogenation coupled with rapid pyrolysis process to give a possibility that converting lignin into not only liquid oil products, but solid hydrogenated lignin can be used to produce more olefins and aromatics in rapid pyrolysis, and presented the advantages and disadvantages of this two-stage process finally.