2 research outputs found
Probing the Lignin Disassembly Pathways with Modified Catalysts Based on Cu-Doped Porous Metal Oxides
Described
are the selectivities observed for reactions of lignin
model compounds with modifications of the copper-doped porous metal
oxide (CuPMO) system previously shown to be a catalyst for lignin
disassembly in supercritical methanol (Matson et al., <i>J. Amer.
Chem. Soc</i>. 2011, 133, 14090â14097). The models studied
are benzyl phenyl ether, 2-phenylethyl phenyl ether, diphenyl ether,
biphenyl, and 2,3-dihydrobenzofuran, which are respective mimetics
of the ι-O-4, β-O-4, 4-O-5, 5-5, and β-5 linkages
characteristic of lignin. Also, briefly investigated as a substrate
is poplar organosolv lignin. The catalyst modifications included added
samariumÂ(III) (both homogeneous and heterogeneous) or formic acid.
The highest activity for the hydrogenolysis of aryl ether linkages
was noted for catalysts with SmÂ(III) incorporated into the solid matrix
of the PMO structure. In contrast, simply adding Sm<sup>3+</sup> salts
to the solution suppressed the hydrogenolysis activity. Added formic
acid suppressed aryl ether hydrogenolysis, presumably by neutralizing
base sites on the PMO surface but at the same time improved the selectivity
toward aromatic products. Acetic acid induced similar reactivity changes.
While these materials were variously successful in catalyzing the
hydrogenolysis of the different ethers, there was very little activity
toward the cleavage of the 5-5 and β-5 C-C bonds that represent
a small, but significant, percentage of the linkages between monolignol
units in lignins
Enhancing Aromatic Production from Reductive Lignin Disassembly: <i>in Situ</i> OâMethylation of Phenolic Intermediates
The selective conversion
of lignin into aromatic compounds has
the potential to serve as a âgreenâ alternative to the
production of petrochemical aromatics. Herein, we evaluate the addition
of dimethyl carbonate (DMC) to a biomass conversion system that uses
a Cu-doped porous metal oxide (Cu<sub>20</sub>PMO) catalyst in supercritical
methanol (sc-MeOH) to disassemble lignin with little to no char formation.
While Cu<sub>20</sub>PMO catalyzes CâO hydrogenolysis of arylâether
bonds linking lignin monomers, it also catalyzes arene methylation
and hydrogenation, leading to product proliferation. The MeOH/DMC
co-solvent system significantly suppresses arene hydrogenation of
the phenolic intermediates responsible for much of the undesirable
product diversity via O-methylation of phenolic âOH groups
to form more stable aryl-OCH<sub>3</sub> species. Consequently, product
proliferation was greatly reduced and aromatic yields greatly enhanced
with lignin models, 2-methoxy-4-propylphenol, benzyl phenyl ether,
and 2-phenoxy-1-phenylethan-1-ol. In addition, organosolv poplar lignin
(OPL) was examined as a substrate in the MeOH/DMC co-solvent system.
The products were characterized by nuclear magnetic resonance spectroscopy
(<sup>31</sup>P, <sup>13</sup>C, and 2D <sup>1</sup>Hâ<sup>13</sup>C NMR) and gas chromatographyâmass spectrometry techniques.
The co-solvent system demonstrated enhanced yields of aromatic products