Lignin First: Confirming the Role of the Metal Catalyst in Reductive Fractionation.

Rhodium nanoparticles embedded on the interior of hollow porous carbon nanospheres, able to sieve monomers from polymers, were used to confirm the precise role of metal catalysts in the reductive catalytic fractionation of lignin. The study provides clear evidence that the primary function of the metal catalyst is to hydrogenate monomeric lignin fragments into more stable forms following a solvent-based fractionation and fragmentation of lignin.

Utility of Core-Shell Nanomaterials in the Catalytic Transformations of Renewable Substrates.

In recent years, core-shell nano-catalysts have received increasing attention due to their tunable properties and broad applications in catalysis. Control of the two components of these materials allows their catalytic properties to be tuned to various sustainable processes in synthetic and energy-related applications. This Concept article describes recent state-of-the-art core-shell materials and their application as heterogeneous catalysts for a range of sustainable catalytic transformations, focusing on two important classes of renewable substrates, CO2 and biomass. In the discussion, emphasis is directed to the role of the constituent parts of the core-shell structure and how they can be manipulated to enhance activity.

Acceptorless dehydrogenation and hydrogenation of N- and O-containing compounds on Pd3Au1(111) facets.

Catalytic dehydrogenation and hydrogenation of amines and alcohols are important in the synthesis of fine chemicals. Despite several efficient homogeneous catalysts having been identified, highly active heterogeneous catalysts remain elusive, although they would meet an unmet need. Here, we show that bimetallic Pd-Au nanoparticles with Pd-to-Au molar ratios of 3:1 immobilized on multiwall carbon nanotubes (Pd3Au1/CNT) display high catalytic activity in the oxidant-free and acceptorless dehydrogenation and hydrogenation of N- and O-containing heterocyclic compounds, amines/imines, and alcohols/ketones. Transmission electron microscopy analysis demonstrates the preferential exposure of Pd3Au1(111) facets on the Pd3Au1/CNT catalyst. Mechanistic insights combining experimental data with density functional theory calculations reveal that the Pd3Au1(111) surface enhances both dehydrogenation and hydrogenation reactions and provides a rationale for the observed enhancements.

Metal-Sulfide Catalysts Derived from Lignosulfonate and their Efficient Use in Hydrogenolysis.

Catalytic lignosulfonate valorization is hampered by the in situ liberation of sulfur that ultimately poisons the catalyst. To overcome this limitation, metal sulfide catalysts were developed that are able to cleave the C-O bonds of lignosulfonate and are resistant to sulfur poisoning. The catalysts were prepared by using the lignosulfonate substrate as a precursor to form well-dispersed carbon-supported metal (Co, Ni, Mo, CoMo, NiMo) sulfide catalysts. Following optimization of the reaction conditions employing a model substrate, the catalysts were used to generate guaiacyl monomers from lignosulfonate. The Co catalyst was able to produce 23.7 mg of 4-propylguaiacol per gram of lignosulfonate with a selectivity of 84 %. The catalysts operated in water and could be recycled and reused multiple times. Thus, it was demonstrated that an inexpensive, sulfur-tolerant catalyst based on an earth-abundant metal and lignosulfonate efficiently catalyzed the selective hydrogenolysis of lignosulfonate in water in the absence of additives.

Indirect CO2 Methanation: Hydrogenolysis of Cyclic Carbonates Catalyzed by Ru-Modified Zeolite Produces Methane and Diols.

We report a ruthenium-modified zeolite which efficiently transforms propylene carbonate to propylene glycol and methane, under solvent-free conditions. The catalyst achieved high product selectivity and no significant ageing effect was observed after multiple cycles. The resulting liquid product (water-containing glycol) can be directly used as anti-freeze solution and the gas phase can directly be used as an energy carrier in the form of H2 -enriched methane. This process efficiently bridges energy storage and an important chemical synthesis under sustainable (CO2 consuming) conditions.

En route to CO2-containing renewable materials: catalytic synthesis of polycarbonates and non-isocyanate polyhydroxyurethanes derived from cyclic carbonates.

Combining CO2-chemistry with biomass conversion allows renewable polymeric materials including polycarbonates and polyhydroxyurethanes (PHUs) to be generated. The demand for robust materials with modular properties that can be prepared on an industrial scale is important and, to date, the most important polymeric materials are derived from petrochemicals. These materials inevitably result in CO2 emissions, and therefore making robust materials from renewable sources will contribute to a more sustainable society. An attractive way to address this challenge is to combine biomass transformations with CO2-fixation and material science. An identified target that combines all three aspects involves the preparation of PHUs (or non-isocyanate polyurethanes, NIPUs) via the polymerization of fully renewable cyclic carbonates derived from biomass and CO2 with a diamine compound that can also been derived from biomass sources. In this review, we critically analyze the progress in catalyst development for the efficient transformation of epoxides and CO2 to cyclic carbonates and polycarbonates. We also discuss the synthesis of PHUs from cyclic carbonates and diamines (not restricted to fully renewable compounds), including challenges in regiocontrol and biodegradability, as well as the role catalysts play in the synthesis of these polymers.

Efficient cleavage of aryl ether C-O linkages by Rh-Ni and Ru-Ni nanoscale catalysts operating in water.

Bimetallic Ru-Ni and Rh-Ni nanocatalysts coated with a phase transfer agent efficiently cleave aryl ether C-O linkages in water in the presence of hydrogen. For dimeric substrates with weaker C-O linkages, i.e. α-O-4 and ß-O-4 bonds, low loadings of the precious metal (Rh or Ru) in the nanocatalysts quantitatively afford monomers, whereas for the stronger 4-O-5 linkage higher amounts of the precious metal are required to achieve complete conversion. Under the optimized, relatively mild operating conditions, the C-O bonds in a range of substituted ether compounds are efficiently cleaved, and mechanistic insights into the reaction pathways are provided. This work paves the way to sustainable approaches for the hydrogenolysis of C-O bonds.

Synthesis of Cross-linked Ionic Poly(styrenes) and their Application as Catalysts for the Synthesis of Carbonates from CO2 and Epoxides.

A series of dicationic styrene-functionalized imidazolium-based salts, in which the two imidazolium rings are bridged by a functionalized spacer, are prepared. The salts are polymerized to afford cross-linked imidazolium-based ionic polystyrene materials, which, owing to the presence of the functionalized spaces, should be highly active organocatalysts for the cycloaddition of CO2 to epoxides to afford cyclic carbonates (CCE reaction). The catalytic activities of the polymers are evaluated in the CCE reaction. The most active catalyst incorporates a diol functionality and is active at 80 °C and a pressure of 4 bar at a loading of 5 mol %, which is comparable to the most active organocatalysts. Moreover, high yields can be obtained under atmospheric pressure upon increasing the temperature to 120 °C. Under harsher conditions, the catalyst is highly active at a loading one order of magnitude lower, highlighting the importance of benchmark conditions for the CCE reaction. Moreover, the polymer catalysts are advantageous because they can be used at low catalyst loadings, the carbonate product is easily isolated in pure form, and loss of activity of the recovered polymer catalyst is not observed during reuse.