科研之友
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摘要
In an effort to augment or displace petroleum as a source of liquid fuels and chemicals, researchers are seeking lower cost technologies that convert natural gas (largely methane) to products such as methanol. Current methane to methanol technologies based on highly optimized, indirect, high-temperature chemistry (>800 degrees C) are prohibitively expensive. A new generation of catalysts is needed to rapidly convert methane and O-2 (ideally as air) directly to methanol (or other liquid hydrocarbons) at lower temperatures (similar to 250 degrees C) and with high selectivity. Our approach is based on the reaction between CH bonds of hydrocarbons (RH) and transition metal complexes, LnM-X, to generate activated LnM-R intermediates while avoiding the formation of free radicals or carbocations. We have focused on the incorporation of this reaction into catalytic cycles by integrating the activation of the CH bond with the functionalization of LnM-R. to generate the desired product and regenerate the LnM-X complex. To avoid free-radical reactions possible with the direct use of O-2, our approach is based on the use of air-recyclable oxidants. In addition, the solvent serves several roles induding protection of the product, generation of highly active catalysts, and in some cases, as the air-regenerable oxidant. We postulate that there could be three distinct classes of catalyst/oxidant/solvent systems. The established electrophilic class combines electron-poor catalysts in acidic solvents that conceptually react by net removal of electrons from the bonding orbitals of the CH bond. The solvent protects the CH3OH by conversion to more electron-poor [CH3OH2](+) or the ester and also increases the electrophilicity of the catalyst by ligand protonation. The nucleophilic class matches electron-rich catalysts with basic solvents and conceptually reacts by net donation of electrons to the antibonding orbitals of the CH bond. In this case, the solvent could protect the CH3OH by deprotonation to the more electron-rich (CH3O](-) and increases the nudeophilicity of the catalysts by ligand deprotonation. The third grouping involves ambiphilic catalysts that can conceptually react with both the HOMO and LUMO of the CH bond and would typically involve neutral reaction solvents. We call this continuum base- or acid-modulated (BAM) catalysis. In this Account, we describe our efforts to design catalysts following these general principles. We have had the most success with designing electrophilic systems, but unfortunately, the essential role of the acidic solvent also led to catalyst inhibition by CH3OH above similar to 1 M. The ambiphilic catalysts reduced this product inhibition but were too slow and inefficient. To date, we have designed new base-assisted CH activation and LnM-R fuctionalization reactions and are working to integrate these into a complete, working catalytic cycle. Although we have yet to design a system that could supplant commercial processes, continued exploration of the BAM catalysis continuum may lead to new systems that will succeed in addressing this valuable goal.
- 出版日期2012-6
