Abstract
More efficient use of lignin carbon is necessary for carbon-efficient utilization of lignocellulosic biomass. Conversion of lignin into valuable aromatic compounds requires the cleavage of C–O ether bonds and C–C bonds between lignin monomer units. The catalytic cleavage of C–O bonds is still challenging, and cleavage of C–C bonds is even more difficult. Here, we report cleavage of the aromatic C–O bonds in lignin model compounds using supported metal catalysts in supercritical water without adding hydrogen gas and without causing hydrogenation of the aromatic rings. The cleavage of the C–C bond in bibenzyl was also achieved with Rh/C as a catalyst. Use of this technique may greatly facilitate the conversion of lignin into valuable aromatic compounds.
Highlights
Of oxidation or depolymerization using acids or bases
An important point about the results of the study was the nature of the aromatic products. 2-Phenethyl phenyl ether could be converted into phenol (40.9 C% yield) and ethylbenzene (23.1 C% yield) using the Pd/C catalyst, the interesting indication being that hydrogenolysis of the C–O ether bonds in 2-phenethyl phenyl ether proceeded without hydrogen gas in supercritical water at 673 K
We have demonstrated that lignin model compounds undergo some unique reactions in which the C–O and C–C bonds are cleaved without adding hydrogen gas and without causing hydrogenation of the aromatic ring
Summary
Of oxidation or depolymerization using acids or bases. Several homogeneous catalysts have recently been investigated for the selective hydrogenolysis of aromatic C–O bonds[21,22,23,24,25]. Sergeev and Hartwig have reported that a nickel carbene complex catalyzes the selective hydrogenolysis of aromatic C–O bonds in Ar–OAr and Ar–OMe (Ar =aryl, Me =methyl) at 393 K with 0.1 MPa hydrogen gas using a strong base, t-BuONa (t-Bu =tert-butyl), and m-xylene as a solvent[23]. Cleavage of C–C bonds, is more difficult because of their high bond-dissociation energy[30] In this communication, we report cleavage of the aromatic C–O bonds in lignin model compounds by the use of supported metal catalysts in supercritical water without the addition of hydrogen gas; cleavage was achieved without causing hydrogenation of the aromatic ring. We selected 2-phenethyl phenyl ether and bibenzyl as model lignin compounds (Fig. 1); these compounds have C–O and C–C bonds characterized by β-O-4 and β- 1 linkages, respectively, between the aromatic lignin units. We found that the kinds of reactions used in this study could be applied to cleavage of the C–C bond in bibenzyl with Rh/C as a catalyst
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