Lignin has potential for a very selective depolymerization leading to a wide spectrum of oxygen-containing aromatics which are difficult to make via existing petrochemical routes (see Figure 8). These products preserve the lignin monomer structure and can be highly desir­able if produced in reasonable quantity with an economic process. The major barrier of this conversion concerns the development of a technology that would allow highly selective bond scissions to maintain the monomeric lignin block structures (Holladay et al., 2007). In addi­tion, proper markets and industrial applications for those aromatics which are related to the original lignin building blocks need to be established. Figure 9 shows the potential reaction

Reducing conditions

FIGURE 9 Potential reaction products from lignin decomposition at different reaction conditions (Haveren et al., 2008).

1. PRINCIPLES OF BIOREFINING

products from the decomposition of lignin via high temperature thermal processes (Haveren et al., 2008). This "cracking" of lignin results in a complex mixture of polyhydroxylated and alkylated phenol compounds, where the abundance and type of products are influenced by reaction conditions. Clearly, improved separation techniques for aromatic lignin monomers must be achieved.