3.4 Catalytic Performance on C-O Bond Cleavage
The catalytic performances of the synthesized catalysts were evaluated by using a typical lignin β-O-4 model compound, 2-phenoxy-1-phenethanol (PPE). Table 1 summarizes the PPE hydrogenolysis results over different catalysts. The reaction does not occur in the absence of catalyst (entry 1) and the NC support also has no catalytic activity in the reaction (entry 2). Ni@NC shows a 31% PPE conversion and a 16% phenol yield (entry 6). In sharp comparison, Ni@N-C SAC gives a much higher PPE conversion (99%) with excellent phenol yield of 92% and ethylbenzene yield of 78%. As a comparison, the commercial Raney nickel exhibits a PPE conversion of 39%, accompanying with phenol and ethylbenzene yields of 24% and 18%, respectively. The above results demonstrate clearly that Ni@N-C SAC possesses high activity in catalyzing PPE hydrogenolysis, during which the C-O bond is cleaved in the presence of H2. Table 1 also shows that the yield of byproduct cyclohexanol (37%) in the reaction over Pd/C catalyst is much higher than that over Ni@N-C SAC (only 1%), indicating that Ni@N-C SAC possesses high selectivity in cleaving C-O bonds without over hydrogenation of aromatic rings.
TABLE 1 Catalytic performance of various catalyst for PPE depolymerizationa.