The first examples of reductive depolymerization of lignin are reported under metal-free conditions. Using hydrosilanes as reductants and B(C 6 F 5 ) 3 as a Lewis acidic catalyst, wood lignin is efficiently converted to a narrow distribution of phenol derivatives, at room temperature. A three-step methodology based on the selection of the wood species and the lignin extraction method, followed by a convergent reductive depolymerization enables the production of four structurally defined aromatic compounds. The phenol products were successfully isolated in 7 to 24 wt% yield from lignin and 0.5 to 2.4 wt% yield from wood. The strategy is found robust and is applied to 15 different wood plants, including gymnosperm and angiosperm species. The efficiency of this novel methodology has been evaluated based on spectroscopic characterization of the lignin preparations and isolated yields of mono-aromatic products.
Producing the next
generation of thermoset polymers
from renewable sources is an important sustainability goal. Hydrogenolysis
of pinewood lignin was scaled
up for the first time from lab scale to a 50 L pilot-scale reactor,
producing a range of depolymerized lignin oils under different conditions.
These lignin hydrogenolysis oils were glycidylated, blended with bisphenol
A diglycidyl ether, and cured to give epoxy thermoset polymers. The
thermal and mechanical properties of the epoxy polymers were assessed
by differential scanning calorimetry, thermogravimetric analysis,
flexural testing, and dynamic mechanical thermal analysis. Replacing
up to 67% of the bisphenol A epoxy with the lignin oil epoxies resulted
in cured epoxy polymers with improvements of up to 25% in flexural
stiffness and strength. Considerable scope exists in simplifying and
scaling up the hydrogenolysis process to produce depolymerized lignins
that can substitute established petrochemicals in the quest for renewable
high-performance thermoset polymers.
The reductive depolymerization of a variety of polymeric materials based on polyethers, polyesters, and polycarbonates is described using hydrosilanes as reductants and metal-free catalysts. This strategy enables the selective depolymerization of waste polymers as well as bio-based polyesters to functional chemicals such as alcohols and phenols at room temperature. Commercially available B(C6 F5)3 and [Ph3 C(+),B(C6 F5)4(-)] catalysts are active hydrosilylation catalysts in this procedure and they are compatible with the use of inexpensive and air-stable polymethylhydrosiloxane and tetramethyldisiloxane as reductants. A significant advantage of this recycling method is derived from its tolerance to the additives present in waste plastics and its ability to selectively depolymerize mixtures of polymers.
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