Department of Chemistry, Northwestern University , Evanston, Illinois 60208-3113, United States.
Acc Chem Res. 2016 May 17;49(5):824-34. doi: 10.1021/acs.accounts.6b00069. Epub 2016 Apr 14.
To reduce global reliance on fossil fuels, new renewable sources of energy that can be used with the current infrastructure are required. Biomass represents a major source of renewable carbon based fuel; however, the high oxygen content (∼40%) limits its use as a conventional fuel. To utilize biomass as an energy source, not only with current infrastructure, but for maximum energy return, the oxygen content must be reduced. One method to achieve this is to develop selective catalytic methods to cleave C-O bonds commonly found in biomass (aliphatic and aromatic ethers and esters) for the eventual removal of oxygen in the form of volatile H2O or carboxylic acids. Once selective methods of C-O cleavage are understood and perfected, application to processing real biomass feedstocks such as lignin can be undertaken. This Laboratory previously reported that recyclable "green" lanthanide triflates are excellent catalysts for C-O bond-forming hydroalkoxylation reactions. Based on the virtues of microscopic reversibility, the same lanthanide triflate catalyst should catalyze the reverse C-O cleavage process, retrohydroalkoxylation, to yield an alcohol and an alkene. However, ether C-O bond-forming (retrohydroalkoxylation) to form an alcohol and alkene is endothermic. Guided by quantum chemical analysis, our strategy is to couple endothermic, in tandem, ether C-O bond cleavage with exothermic alkene hydrogenation, thereby leveraging the combined catalytic cycles thermodynamically to form an overall energetically favorable C-O cleavage reaction. This Account reviews recent developments on thermodynamically leveraged tandem catalysis for ether and more recently, ester C-O bond cleavage undertaken at Northwestern University. First, the fundamentals of lanthanide-catalyzed hydroelementation are reviewed, with particular focus on ether C-O bond formation (hydroalkoxylation). Next, the reverse C-O cleavage/retrohydroalkoxylation processes enabled by tandem catalysis are discussed for both ether and ester C-O bond cleavage, including mechanistic and computational analysis. This is followed by recent results using this tandem catalytic strategy toward biomass relevant substrates, including work deconstructing acetylated lignin models, and the production of biodiesel from triglycerides, while bypassing the production of undesired glycerol for more valuable C3 products such as diesters (precursors to diols) in up to 47% selectivity. This Account concludes with future prospects for using this tandem catalytic system under real biomass processing conditions.
为了减少对化石燃料的全球依赖，需要寻找新的可再生能源，这些能源可以与现有的基础设施一起使用。生物质代表了可再生碳基燃料的主要来源；然而，其高氧含量（约 40%）限制了其作为传统燃料的使用。为了利用生物质作为能源，不仅要利用现有的基础设施，还要最大限度地回收能源，就必须降低其氧含量。实现这一目标的一种方法是开发选择性催化方法，以裂解生物质中常见的 C-O 键（脂肪族和芳香族醚和酯），最终以挥发性 H2O 或羧酸的形式去除氧。一旦理解并完善了 C-O 键裂解的选择性方法，就可以应用于处理真正的生物质原料，如木质素。本实验室之前曾报道，可回收的“绿色”镧系元素三氟甲磺酸酯是 C-O 键形成氢烷氧基化反应的优良催化剂。基于微观可逆性的优点，相同的镧系元素三氟甲磺酸酯催化剂应该催化 C-O 键断裂的逆反应，即反氢烷氧基化反应，生成醇和烯烃。然而，醚 C-O 键的形成（反氢烷氧基化）形成醇和烯烃是吸热的。受量子化学分析的指导，我们的策略是将吸热的串联醚 C-O 键断裂与放热的烯烃加氢反应偶联，从而在热力学上利用组合催化循环形成整体有利的 C-O 键断裂反应。本综述回顾了在西北大学进行的热力学偶联催化在醚和最近酯的 C-O 键断裂方面的最新进展。首先，综述了镧系元素催化的氢元素化的基本原理，特别关注醚 C-O 键的形成（氢烷氧基化）。接下来，讨论了通过串联催化实现的 C-O 键断裂/反氢烷氧基化过程，包括机理和计算分析。接下来是使用这种串联催化策略对与生物质相关的底物的最新结果，包括对乙酰化木质素模型的解构以及从三酸甘油酯生产生物柴油的工作，同时避免了生产不受欢迎的甘油，以获得更有价值的 C3 产物，如二酯（二醇的前体），选择性高达 47%。本综述最后展望了在实际生物质处理条件下使用这种串联催化系统的未来前景。
