EAGER: Biomimetic Catalysts For Selective Methane Oxidation

摘要#1349713Lo，Cynthia S.成功地将甲烷氧化升级为甲醇将同时利用丰富的页岩气储量，减少人类对传统化石燃料的依赖，并以有利的方式减少温室气体排放。作为优选的氧化目标，甲醇是非常通用的，因为它可以转化为各种烃，并保留传统化石燃料的大部分能量密度。不幸的是，甲烷氧化转化的关键问题与选择性有关，而不是反应性;很难避免完全氧化为一氧化碳或二氧化碳。为了部分氧化甲烷，催化剂必须设计成不利于包括全部氧化过程的基本步骤，或者选择性地仅释放必需量的氧以产生所需产物。密苏里州圣路易斯市华盛顿大学的Cynthia Lo教授假设氧化铜/氧化石墨烯复合材料将是选择性甲烷氧化为甲醇的有效催化剂。拟议的催化剂设计中的活性位点模拟MMO酶中的活性位点，MMO酶中含有稳定甲烷衍生物的氧桥铜（II）络合物。n-型氧化石墨烯用作该反应的氧源，p-型氧化铜用于稳定复合材料。催化剂复合物的光化学活化将用于实现甲烷选择性氧化成甲醇。产物分布应该是石墨烯表面上氧含量的单调函数。催化剂应在空气中氧化再生。凭借美国国家科学基金会催化生物催化计划颁发的EAGER奖，PI计划在流通式反应器中实验验证催化剂设计，并将性能与第一原理计算产生的预测进行比较。这需要独特的反应器设计，以促进甲烷选择性氧化为增值化学品和燃料。新的设计是一个流通，单片光纤光反应器，以适应这里提出的催化剂系统。催化剂设计的逻辑在于利用氧化铜/氧化石墨烯复合物作为温和的氧化剂应降低甲烷完全氧化成二氧化碳的倾向。与在标准氧化反应中在升高的温度下观察到的多个C-H键解离相比，使用光化学活化应促进甲烷中仅一个C-H键的断裂。这些概念的整合代表了催化剂设计和反应工程的潜在变革性方法，其中催化剂提供反应物之一（即，氧气）用于该方法中;实际上，该反应最终可以结合到化学循环技术中。这一提议的更广泛影响是在两个领域推进预期的社会成果：减缓温室气体排放，以及创造一种经济上可行的从传统化石燃料向替代能源技术过渡的手段。由于甲烷的全球变暖潜力约为二氧化碳的25倍，因此对碳中和且不产生二氧化碳副产品的甲烷转化技术进行基础科研是合乎逻辑的。学术界和工业界应该投入更多的努力，将甲烷升级为增值的化学品和燃料，并关闭碳循环。此外，用于合成催化剂和建造反应器的所有材料都由廉价和地球丰富的元素组成，这是可持续性的最佳例子。该项目适合于EAGER资助机制，因为它涉及与目前旨在甲烷转化的研究工作完全不同的方法。通过在多个长度和时间尺度上对催化剂结构-性能-反应性关系进行更全面的了解，将开始有效利用丰富而廉价的甲烷储量，并为实现能源独立奠定基础。罗教授的技巧将用于检验本EAGER中的假设。在华盛顿大学的过去六年里，她在计算催化方面积累了丰富的专业知识，阐明了金属氧化物催化剂的结构性质关系，并证明了甲烷活化和脱氢在催化剂相之间的界面进行（例如，金属载体），特别是当存在位置缺陷时。这支持了本研究中提出的催化剂假设。PI计划继续在当地K-12社区的科学推广方面的现有努力，通过与圣路易斯市KIPP的未得到充分服务的中学生合作，他们正在为密苏里州科学奥林匹克竞赛做准备;这项工作自2011年以来一直在进行，并成功地促进了STEM学习。参加的中学生将被邀请参观华盛顿大学的PI实验室，并观察反应堆的演示。

Abstract#1349713Lo, Cynthia S.Successful oxidative upgrading of methane to methanol would simultaneously utilize abundant shale gas reserves, reduce human reliance on traditional fossil fuels, and mitigate greenhouse gas emissions in a thermodynamically favorable manner. As the preferred oxidation target, methanol is quite versatile since it can be converted to a variety of hydrocarbons and retain much of the energy density of traditional fossil fuels. Unfortunately, the key issue with the oxidative conversion of methane is related to selectivity, rather than reactivity; it is difficult to avoid total oxidation to carbon monoxide or carbon dioxide. To partially oxidize methane, catalysts must be designed that either disfavor the elementary steps comprising the total oxidation process, or selectively release only a requisite amount of oxygen to produce the desired products. Prof. Cynthia Lo from Washington University, St. Louis, MO hypothesizes that copper oxide/graphene oxide composites will be effective catalysts for the selective methane oxidation to methanol. The active sites in the proposed catalyst design mimic the active sites in the MMO enzyme, which contains oxo-bridged copper (II) complexes that stabilize methane derivatives. n-type Graphene oxide serves as the source of oxygen for this reaction, with p-type cupric oxide used to stabilize the composites. Photochemical activation of the catalyst composite will be used to achieve selective methane oxidation to methanol. The product distribution should be a monotonic function of the oxygen content on the graphene surface. The catalyst should be regenerated by oxidation in air. With this EAGER award made by the Catalysis & Biocatalysis Program of the National Science Foundation, the PI plans to experimentally validate the catalyst design in a flow-through reactor and compare performance to the predictions generated from first-principles calculations. This requires a unique reactor design that facilitates selective methane oxidation to value-added chemicals and fuels. The new design is for a flow-through, monolithic optical fiber photoreactor necessary to accommodate the catalyst system proposed here. The logic of the catalyst design rests on utilizing the cupric oxide/graphene oxide composite as a mild oxidant should reduce the propensity of methane to completely oxidize to carbon dioxide. Using photochemical activation should facilitate the breaking of only one C-H bond in methane, compared to the multiple C-H bond dissociations observed at elevated temperatures in standard oxidation reactions. The integration of these concepts represents a potentially transformative approach to catalyst design and reaction engineering, where the catalyst provides one of the reactants (i.e., oxygen) for use in the process; indeed, this reaction may ultimately be incorporated into a chemical looping technology. The broader impact of this proposal is to advance desired societal outcomes in two areas: the mitigation of greenhouse gas emissions, and the creation of an economically-viable means of transitioning from traditional fossil fuels to alternative energy technologies. As the global warming potential of methane is approximately 25 times higher than that of carbon dioxide, pursuit of basic scientific research on methane conversion technologies that are carbon neutral and do not produce carbon dioxide as byproducts is logical. More academic and industrial effort should be devoted to upgrading methane to value-added chemicals and fuels, and closing the carbon cycle. In addition, all materials used to synthesize the catalysts and construct the reactor are composed of inexpensive and earth-abundant elements, in the best example of sustainablility. This project is appropriate for the EAGER funding mechanism, in that it involves a radically different approach to the current research efforts aimed at methane conversion. By developing a more complete understanding of catalyst structure-property-reactivity relationships across multiple length and time scales, a start will be made on achieving effective utilization of the abundant and inexpensive methane reserves, and lay the path towards energy independence. The skills of Prof. Lo will be utilized in testing the hypotheses in this EAGER. Over the past six years at Washington University, she has developed significant expertise in computational catalysis elucidating structure property relationships in metal oxide catalysts, and demonstrating that methane activation and dehydrogenation proceeds at the interface between catalyst phases (e.g., metal-support) particularly when site defects are present. This supports the catalyst hypothesis presented in this study. The PI plans to continue existing efforts in science outreach to the local K-12 community, by working with underserved middle school students at KIPP: Inspire Academy in St. Louis City that are preparing for the Missouri Science Olympiad Competition; this effort has been ongoing since 2011, and has successfully promoted STEM learning. The participating middle school students will be invited to visit the PIs laboratory at Washington University and observe a demonstration of the reactor.