Green Routes to Fluorocarbons

氟碳化合物是各种应用的重要材料。例如，小分子氢氟碳化合物（hfc）具有较低的臭氧消耗潜力，并已取代禁用的氯氟碳化合物（CFCs），作为制冷剂、发泡剂、溶剂等。最近，不饱和氢氟碳化合物正在上市，相对于其饱和类似物，它们大大降低了全球变暖潜力。然而，这些分子的工业生产包括使用对环境有问题的氯碳化合物和有毒金属，如锑。含氟聚合物的独特性能，如低表面能，高热稳定性和化学稳定性，使其特别适用于热塑性塑料，涂料，弹性体膜和功能材料。不幸的是，这些材料是通过使用环境持久性氟表面活性剂的能源密集型自由基工艺或乳液聚合制备的。因此，对于重要的单体，如偏氟乙烯（CH2=CF2）和六氟丙烯[CF2=CF(CF3)]，无法控制插入区域和立体选择性，从而无法控制聚合物的微观结构。此外，由这些单体与四氟乙烯（CF2=CF2）共聚而得到的重要材料也由于目前采用的聚合方法的随机性而缺乏一致的性能。在我们的研究计划中，我们的目标是：1)使用商业催化热解装置，在低压（< 20 torr）下，通过廉价的散装聚四氟乙烯（PTFE）和二氟甲烷，乙炔或合成气（CO/H2）共热解制备氢氟烯烃（hfa）。含碳气体经多相催化剂活化后，与PTFE热解生成的气相CF2结合。固体碱如MgO或水滑石将从CH2F2中去除HF生成CHF碳烯，金属催化剂将激活乙炔使CF2加成或插入到C-H键中，费托催化剂将在催化剂表面生成CHn单元，与气相CF2碳烯反应。优化的反应条件将用于选定的hfa的规模化生产，供实验室使用，并随后将技术转让给工业合作伙伴。

Fluorocarbons are important materials for a variety of applications. Small molecule hydrofluorocarbons (HFCs), for example, have a low ozone depletion potential and have replaced banned chlorofluorocarbons (CFCs) as refrigerants, blowing agents, solvents, etc. More recently, unsaturated HFCs are being marketed that have greatly diminished global warming potential relative to their saturated analogs. Industrial production of these molecules, however, includes use of environmentally problematic chlorocarbons and toxic metals such as antimony. The unique properties of fluoropolymers such as low surface energy, high thermal and chemical stability make them especially useful as thermoplastics, coatings, elastomer membranes and functional materials. Unfortunately, these materials are prepared by energy-intensive radical processes or emulsion polymerization that employs environmentally persistent fluorosurfactants. As a result, for important monomers such as vinylidene fluoride (CH2=CF2) and hexafluoropropene [CF2=CF(CF3)] there is no way to control the insertion regio- and stereoselectivity and thus the polymer microstructure. Moreover, important materials derived from copolymerization of these monomers with tetrafluoroethylene (CF2=CF2) also suffer from a lack of consistent properties resulting from the random nature of the currently employed polymerization methods. In our research program we aim to: 1) Prepare hydrofluoroalkenes (HFAs) from co-pyrolysis of inexpensive bulk polytetrafluoroethylene (PTFE) and difluoromethane, acetylene or syngas (CO/H2) at low pressures (< 20 torr) using a commercial catalytic pyrolysis unit. The carbon-containing gases are activated by the heterogeneous catalyst and then combine with gas phase CF2 generated by the PTFE pyrolysis. Solid bases such as MgO or hydrotalcite will be used to remove HF from CH2F2 to generate CHF carbene, metal catalysts will activate acetylene for CF2 addition or insertion into a C-H bond, and Fisher-Tropsch catalysts will be used to generate CHn units on the catalyst surface for reaction with the gas phase CF2 carbenes. Optimized reaction conditions will be used for scaled-up production of selected HFAs for lab use and subsequent technology transfer to industry partners. 2) Develop base metal complex-catalyzed reactions of HFAs, proceeding through metallacyclopentane intermediates, that will offer greener routes to technologically valuable HFCs. Our promising results with low-coordinate organofluoronickelacycles will be extended to include hemilabile N-heterocyclic carbene-thioether ligands for stabilization of the low-valent catalyst under H2 and bifunctional P-N and P-B ligands for heterolytic cleavage of dihydrogen and reversible C-F bond activation, respectively. Low-coordinate Fe complexes will also be evaluated to compare the d5,6 organofluorometallacycle reactivity with our d7,8 nickel examples, particularly with regard to formation and functionalization of mixed HC/FC metallacycles and development of tandem processes that make productive use of the HF product. 3) Investigate nucleophilic metal fluorocarbenes as initiators for HFA metathesis and polymerization via metallacyclobutane intermediates. Building on newly developed CF2 transfer to low-valent metals using Me3SiCF3 and catalytic NaI, we will synthesize a number of new nucleophilic base metal fluorocarbenes and investigate their cycloaddition reactivity with HFAs and subsequent fluoride abstraction. Target systems include XL2 pincer ligands and metal hydrofluorocarbenes. These fundamental studies will be accompanied by rapid throughput catalytic testing of tandem processes using the silyl reagent to deliver CFRF and CHRF carbenes to low-valent metal precursors and subsequent catalyzed insertion into C-H bonds.