481 / 2018-09-09 10:22:59

Depolymerization and in situ hydrodeoxygenation of pyrolytic lignin in supercritical methanol with Cu-Mg-Al mixed oxide catalysts

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Supercritical fluids are good media for biomass conversion, and related researches are very common, in which lignin degradation is one of the major parts. However, lignin fragments used in these researches were mainly technical lignin and original lignin in biomass. Conversion and upgrading of pyrolytic lignin (PL) in supercritical fluids is rarely reported. Therefore, we investigated the depolymerization and hydrodeoxygenation (DHDO) of PL in supercritical methanol (SCM) with Cu-Mg-Al mixed oxide catalysts for the first time. Results showed that after PL SCM-DHDO, esters, cyclohexanols, aromatics, as well as their methylated derivatives were produced. After SCM-DHDO with 2 h, the total carbon yields of monomers reached to the maximum, up to 19.77 C%. Esters were generated from the esterification of residual fatty acids with methanol. Monomeric aromatics benefited from the degradation of PL molecules. Whereas cyclohexanols were attributed to the hydrogenation of PL and/or produced aromatics. Methylation contributed to those methylated derivatives. All these reactions occurred in PL SCM-DHDO were further confirmed with quantitative 13C nuclear magnetic resonance (NMR). As displayed in Fig. 1, with reaction time extending, the percentage of aromatics kept decreasing from 67.2% to 27.1%, whereas the percentage of C-C aliphatics increased continuously from 15.9% to 64.2%. There was no doubt that this revealed the saturation of the aromatic rings. As for C-O aliphatics, its percentage dropped slowly from 12.8% to 8.0%. This was quite different from the general lignin hydrogenation, in which the percentage of C-O aliphatics rose. This illustrated that deoxygenation and hydrogenation were carried out simultaneously during PL SCM-DHDO. Two-dimensional (2D) heteronuclear singular quantum correlation (HSQC) and heteronuclear multiple bond correlation (HMBC) NMR were further programmed for detailed reactions on specific positions. Overall, PL was well degraded in SCM-DHDO with Cu-Mg-Al mixed oxide catalysts, meanwhile produced products were in situ upgraded with hydrodeoxygenation.