BTX from Lignin

Abstract

Benzene, toluene, and xylenes (BTX) represent three of six basic petroleum hydrocarbons produced on a worldwide basis. The annual demand for BTX is nearly 108 million metric tonnes at a demand growth rate of approximately 5%. Shale oil-sourced petroleum produces a lighter naphtha, which will reduce reformer BTX yields. Depleted crude oil stocks and consumers eagerness to substitute petroleum-based incumbent hydrocarbons with biobased substitutes have created a tremendous opportunity to upgrade very low valued, natural aromatic polymer in lignin. A major interest is now toward developing a lignin to BTX process. Technical lignin is abundantly available because of its production from the very large-scale pulp and paper process. However, not all technical lignin is upgradable because some of them contain very high concentration of other materials that cannot be tolerated in upgrading process. The lignin to BTX process is composed of two main steps: first is lignin depolymerization to aromatic constituents (mostly phenolics), and these oxygenated aromatic monomers are then catalytically hydrodeoxygenated/upgraded to bio-oil that can be refined to bio-BTX. Thermolysis (pyrolysis), gasification, oxidative, alkaline, and reductive lignin depolymerization are options. Any of these depolymerization can occur in the presence of a catalyst. The most frequently investigated depolymerization process is fast pyrolysis, and this process can employ a catalyst, typically a zeolite (a.k.a. catalytic fast pyrolysis). Hydrodeoxygenation in the second step can employ a broad range of catalysts that can include zeolites, metal oxides, and supported metals (precious and base). In both of these steps, significant challenges increase risks of commercialization: severe fouling/deactivation of catalyst in the depolymerization, repolymerization of aromatic monomeric species to char, and formation of broad range of products from hydrodeoxygenations, necessitating costly separation processes. Countermeasures to overcome these challenges are also discussed.