化学新物种 ： 质子化甲烷 CH 5 + 诸平 甲烷CH 4 大家很熟悉，中学化学中就已经了解了它的制备和性质。它既是最简单的有机化合物，也是最稳定的化合物，因为CH 4 是由碳sp 3 杂化轨道和氢s轨道成键而形成的四面体结构。但是，由斯蒂芬·施莱默（ StephanSchlemmer）教授领导的科隆大学（ University ofCologne）的一个科学家小组， 2015年 3月 20日在《科学》（ Science ） 杂志网站发表了他们的研究成果，成功地解释了 CH 5 + 分子离子的高度流动光谱。 CH 5 + 分子离子的形成反应可以表示如下： CH 4 +H + =CH 5 + 但是原子、离子之间究竟是如何成键的，需要看看相关报道或许能够得到答案。 Science 20 March 2015: Vol. 347 no. 6228 pp. 1313-1314 DOI: 10.1126/science.aaa6935 Taming CH 5 + , the “enfant terrible” of chemical structures Takeshi Oka Department of Chemistry and Department of Astronomy and Astrophysics, The Enrico Fermi Institute, University of Chicago, Chicago, IL 60637, USA. E-mail: t-oka{at}uchicago.edu Protonated methane, CH 5 + , is a quantum dynamical system that challenges our understanding of chemical bonding and structure. The bonding does not lead to a trigonal bipyramid. Instead, the five protons swarm around the central carbon, and this gives rise to an incredibly complex vibration-rotation-tunneling infrared spectrum ( 1 ), making it an “enfant terrible” for spectroscopists. Ab initio theory has found that “there is essentially no barrier to hydrogen scrambling” ( 2 ) and “the very concept of molecular structure becomes problematic for this molecule” ( 3 ). For its parent molecule, CH 4 , each rotational level corresponds to one quantum state, but for CH 5 + it corresponds to 2 × 5! = 240 states. However, on page 1346 of this issue, Asvany et al. ( 4 ) report combination differences (Co-Diffs) of the low-energy levels of CH 5 + , a first step at “taming” its spectrum. The taming of the shrew: Scientists decipher the spectrum of CH 5 + for the first time The CH5+ molecular ions were investigated in an ion trap setup such as this. The trap itself is the illuminated cylindrical cavity in the middle of the photo, in which some trap electrodes can be seen. In this trap, several thousand CH5+ ions were stored, cooled, and investigated by a laser. The obtained high-resolution spectra are shown in the inset, as well as a sketch of the CH5+ molecule in motion. Credit: Debora Schiffer/Oskar Asvany, University of Cologne For the first time ever, a team of scientist from the University of Cologne headed by Professor Stephan Schlemmer succeeded in understanding the spectrum of the highly fluxional molecule CH 5 + . This insight, gained in collaboration with a Japanese colleague, was made possible by the extreme cooling of this enigmatic molecule and a highly accurate measurement of its vibrational transitions. The results will be presented on March 20, 2015 in Science magazine. CH 5 + , formed by adding a proton (H + ) to the well-known methane (CH 4 ) molecule, is the prototype of fluxional molecules. In contrast to common molecules, which are depicted as a rigid structure consisting of balls (atoms) and sticks (chemical bonds), the five hydrogen nuclei in CH 5 + can move quite freely around the carbon nucleus . It is thus constantly in motion, even at an extremely low temperature. Bonds are broken and reformed all the time, and therefore the simple model of balls and sticks does not apply. There has thus been a long debate whether CH 5 + has a structure at all. This extraordinary fluxional behavior is reflected in the spectra of CH 5 + . Usually such spectra are recorded in the lab to characterize and identify molecules. With the help of suitable theoretical models , the vibrational spectra can yield information about bond strengths and molecular structure. For CH 5 + , however, the hitherto known spectra have been so chaotic that not a single of the many hundred vibrational transitions could be understood or assigned. This has been considered one of the last mysteries of molecular physics. By developing and applying new ion trap experiments, physicists from the University of Cologne have now succeeded in storing a pure sample of CH 5 + ions and cooling it down to a temperature close to absolute zero. With the help of a so-called frequency comb, the vibrational transitions could be measured with high accuracy, leading to a reconstruction of the lowest energy levels. This very technical approach was necessary due to the complete lack of theoretical models for this exceptional molecule. The results are thus based only on the experimental data and the fundamental principle of quantum mechanics, according to which the observed vibrational transitions are based on a scheme of discrete energy levels. This animation illustrates the extreme fluxionality of the CH 5 + molecule. The black ball in the middle is the carbon nucleus, and the red and white balls are hydrogen nuclei. The blue clouds symbolize the binding electron pairs. Credit: Dominik Marx, Ruhr-Universität Bochum Surprisingly, the results are in accordance with the simple notion that the five hydrogen nuclei can move quite freely around the central carbon nucleus, with their distance to it being more or less fixed. Whether this simple picture is valid will have to be tested in further investigations. In any case, the highly accurate data will challenge future theoretical models to interpret the discovered energy levels . The entire class of fluxional molecules will profit from these developments. Explore further: Uncovering the forbidden side of molecules More information: Taming CH 5 + , the enfant terrible of chemical structures, Science , www.sciencemag.org/lookup/doi/… 1126/science.aaa6935
新闻报道见此： http://www.cas.cn/xw/cmsm/201405/t20140513_4118737.shtml 《Science》编辑的总结： EDITOR'S SUMMARY Upgrading Methane Sans Oxygen Direct routes to converting methane to higher hydrocarbons can allow natural gas to be used to provide chemical feedstocks. However, the reaction conditions needed to activate the strong C-H bond tend to overoxidize the products. Guo et al. (p. 616) report a high-temperature nonoxidative route that exposes methane to isolated iron sites on a silica catalyst. Methyl radicals were generated and coupled in the gas phase to form ethylene and aromatics along with hydrogen. The isolation of the active sites avoided surface reactions between the radicals that would deposit solid carbon. 补充材料有对催化剂合成和反应的描述： http://www.sciencemag.org/content/suppl/2014/05/07/344.6184.616.DC1/Guo.SM.pdf 文章让人很兴奋，如果这个技术真的产业化了，对天然气的利用绝对是革命性的突破。但是冷静下来可以想到这么好的技术或许不会先发paper，所以不禁还是有些疑问，这些疑 问是： 一、只要催化剂中没有成簇的金属，就可以防止积碳？实际情况不是如此。现在在线测量甲烷氢同位素的方法是把甲烷在1350摄氏度（比这篇文章的反应温度高之有限）的活性炭上彻底分解为氢气和碳；而活性炭是担载在氧化铝上面，这个反应并不需要金属催化。 二、乙烯是个热力学很不稳定的物质，高温下远远比甲烷容易分解为石墨和氢气，而且 乙烯更容易和自由基反应 。感觉上 乙烯应该是浓度很低的暂态产物， 但是产物中 乙烯浓度很高。 补充材料页5中把 乙烷作为中间体，而产物中并没有乙烷的选择性，很让人费解。这些结果无论 从热力学还是动力学上都不好理解。 三、如果产物中有少量积碳，如何能够定量。 所以目前对这个潜在技术至多只能谨慎乐观，如果能走到中试，或许会回答上面问题。
ScienceDaily ( 科学日报 ) 2012 年 8 月 9 日 报道了意大利 的里雅斯特大学（ University of Trieste ）、西班牙加迪斯大学（ Universidad de Cádiz ）以及美国宾夕法尼亚大学（ University of Pennsylvania ）的研究人员合作，研发出一种更廉价、更清洁的甲烷燃烧催化剂。因为随着 全球石油储量的减少 , 天然气已经成为越来越重要的能源之一。天然气的主要成分是甲烷 , 甲烷与许多其他的碳氢化合物相比，其优势是完全燃烧之后释放的二氧化碳更少。但由于甲烷分子的结构非常稳定 , 很难释放出存储的能量；而未燃烧的甲烷会逃逸到大气中 , 其温室效应是二氧化碳的 20 倍。现在 , 来自美国宾夕法尼亚大学、意大利的里雅斯特大学、西班牙加迪斯大学的研究人员一起 , 创造出一种可以催化甲烷燃烧的催化剂，这种材料的催化效率是目前可用的催化剂的 30 倍。 这一研究成果为更彻底的开发利用甲烷作为能源之一提供了一种有效途径 , 可降低燃气车辆的温室气体排放。这种催化剂也可提供一种更干净、更便宜的燃气涡轮机催化燃烧获得能量的方式。宾夕法尼亚大学化学和生物分子工程学系的 Raymond J. Gorte 教授对此评价说：“很难想出有这样的材料 , 它们具有足够的活性和稳定性以承受甲烷燃烧的恶劣条件 , 我们的材料对于一些重要的应用而言，看起来很有希望。” 现在宾夕法尼亚大学化学系的一个博士后研究员 Matteo Cargnello 也加入 Raymond J. Gorte 教授的研究小组，还有 Raymond J. Gorte 教授实验室毕业的博士生 Kevin Bakhmutsky 。他们的合作者包括意大利的里雅斯特大学的 Paolo Fornasiero 和 Tiziano Montini ；以及西班牙加的斯大学的 Jos é J. Calvino, Juan Jos é Delgado 和 Juan Carlos Hern á ndez Garrido 。这项研究 2012 年 8 月 10 日 发表在《科学》（ Science ）杂志网站—— M. Cargnello, J. J. Delgado Jaén, J. C. Hernández Garrido, K. Bakhmutsky, T. Montini, J. J. Calvino Gámez, R. J. Gorte, and P. Fornasiero. Exceptional Activity for Methane Combustion over Modular Pd@CeO 2 Subunits on Functionalized Al 2 O 3 . Science , August 2012: 713-717 DOI: 10.1126/science.1222887 . Matteo Cargnello 作为这篇论文的第一作者，他开始做这个项目是在的里雅斯特大学就读本科专业时 , 在访问 Raymond J. Gorte 实验室期间 , 继续合作同时他还在意大利的里雅斯特大学纳米技术研究生院在攻读博士学位。催化剂使化学变换的更快 , 更简单 , 更节能 , 常常更安全的一些材料。例如 , 一辆汽车的催化转换器就是要将排放废气转换成无害的产物。然而 , 目前使用的甲烷燃烧催化剂 , 并不能完全将甲烷彻底催化转化为 CO 2 和 H 2 O, 总有一部分未燃烧的甲烷逃逸到大气之中 , 并导致气候变化。特别是如果你有一台天然气发动机 , 那么排气管排出的主要成分就是尚未燃烧的甲烷气。此外 , 这些传统催化剂可以要求温度高达 600 -700 ℃ 促进 反应进行。但是 , 催化剂本身也往往会因为甲烷燃烧所产生的高温而失活。 当以甲烷作为能源用于燃气涡轮发动机时，会导致额外的环境损害。在这个过程中 , 甲烷是通常在非常高的温度下燃烧 , 甚至超过 800 ℃ 。 当温度上升到大约 1300 ℃ 或更高时，反应就能够产生人体有害的副产品 , 包括氧化氮、硫氧化物和一氧化碳。 甲烷燃烧的传统催化剂是由金属纳米颗粒组成 , 特别是钯 , 沉积在氧化物如氧化铈 (CeO 2 ) 等的表面。研究人员对于这种方法进行了微调 , 主要是依赖于纳米颗粒的自组装。他们首先建立了直径只有 1.8 nm 的钯微粒，然后由 CeO 2 制成的保护多孔壳围在钯微粒周围 , 创建了一堆具有金属核心的球形结构体。因为这些小颗粒在加热时成团积聚 , 而且这些成团积聚的块体可以降低催化剂活性 , Raymond J. Gorte 教授领导的研究团队将其沉积于由氧化铝组成的疏水性表面 , 以确保它们是均匀分布的。 这些技术是纳米技术中最普通的 , 但这对于制造催化剂材料来说是一种新方法。测试材料的活性之后研究人员发现 , 他们的核 - 壳型纳米结构甲烷燃烧催化剂，其性能优于目前使用的最好的甲烷燃烧催化剂 30 倍，而且使用金属数量相同。催化使甲烷完全燃烧温度为 400 ℃ 。这种催化剂不仅使甲烷充分燃烧，提高燃气涡轮机的燃料利用效率，而且可以有效控制汽车尾气造成的大气污染。研究人员计划进一步研究新催化剂的结构来更好地理解为什么它是如此有效。他们将使用类似的方法来创建新材料来进行测试。也可以使用此组装法去测试对不同类型的金属和氧化物 , 这将有可能会使研究人员制备一系列新型催化剂材料 , 其中一些可能是除了甲烷燃烧催化反应之外的很好的催化剂。 更多信息请浏览原文： http://www.sciencemag.org/content/337/6095/713
2010年美国墨西哥湾深海油气井泄漏事件想毕大家还记忆犹新。其实这类灾害与深海油气井在海底建造的天然气水合物有关。最近MIT科学家研究出抑制深海油气井甲烷水合物建造的表面涂层，值得我们关注。（ http://web.mit.edu/newsoffice/2012/undersea-ice-clog-mitigation-0412.html ） During the massive oil spill from the ruptured Deepwater Horizon well in 2010, it seemed at first like there might be a quick fix: a containment dome lowered onto the broken pipe to capture the flow so it could be pumped to the surface and disposed of properly. But that attempt quickly failed, because the dome almost instantly became clogged with frozen methane hydrate. Methane hydrates, which can freeze upon contact with cold water in the deep ocean, are a chronic problem for deep-sea oil and gas wells. Sometimes these frozen hydrates form inside the well casing, where they can restrict or even block the flow, at enormous cost to the well operators. Now researchers at MIT, led by associate professor of mechanical engineering Kripa Varanasi, say they have found a solution, described recently in the journal Physical Chemistry Chemical Physics . The paper’s lead author is J. David Smith, a graduate student in mechanical engineering. The deep sea is becoming “a key source” of new oil and gas wells, Varanasi says, as the world’s energy demands continue to increase rapidly. But one of the crucial issues in making these deep wells viable is “flow assurance”: finding ways to avoid the buildup of methane hydrates. Presently, this is done primarily through the use of expensive heating systems or chemical additives. “The oil and gas industries currently spend at least $200 million a year just on chemicals” to prevent such buildups, Varanasi says; industry sources say the total figure for prevention and lost production due to hydrates could be in the billions. His team’s new method would instead use passive coatings on the insides of the pipes that are designed to prevent the hydrates from adhering. These hydrates form a cage-like crystalline structure, called clathrate, in which molecules of methane are trapped in a lattice of water molecules. Although they look like ordinary ice, methane hydrates form only under very high pressure: in deep waters or beneath the seafloor, Smith says. By some estimates, the total amount of methane (the main ingredient of natural gas) contained in the world’s seafloor clathrates greatly exceeds the total known reserves of all other fossil fuels combined. Inside the pipes that carry oil or gas from the depths, methane hydrates can attach to the inner walls — much like plaque building up inside the body’s arteries — and, in some cases, eventually block the flow entirely. Blockages can happen without warning, and in severe cases require the blocked section of pipe to be cut out and replaced, resulting in long shutdowns of production. Present prevention efforts include expensive heating or insulation of the pipes or additives such as methanol dumped into the flow of gas or oil. “Methanol is a good inhibitor,” Varanasi says, but is “very environmentally unfriendly” if it escapes. Varanasi’s research group began looking into the problem before the Deepwater Horizon spill in the Gulf of Mexico. The group has long focused on ways of preventing the buildup of ordinary ice — such as on airplane wings — and on the creation of superhydrophobic surfaces, which prevent water droplets from adhering to a surface. So Varanasi decided to explore the potential for creating what he calls “hydrate-phobic” surfaces to prevent hydrates from adhering tightly to pipe walls. Because methane hydrates themselves are dangerous, the researchers worked mostly with a model clathrate hydrate system that exhibits similar properties. The study produced several significant results: First, by using a simple coating, Varanasi and his colleagues were able to reduce hydrate adhesion in the pipe to one-quarter of the amount on untreated surfaces. Second, the test system they devised provides a simple and inexpensive way of searching for even more effective inhibitors. Finally, the researchers also found a strong correlation between the “hydrate-phobic” properties of a surface and its wettability — a measure of how well liquid spreads on the surface. The basic findings also apply to other adhesive solids, Varanasi says — for example, solder adhering to a circuit board, or calcite deposits inside plumbing lines — so the same testing methods could be used to screen coatings for a wide variety of commercial and industrial processes. Richard Camilli, an associate scientist in applied ocean physics and engineering at Woods Hole Oceanographic Institution who was not involved in this study, says, “The energy industry has been grappling with safety and flow-assurance issues relating to hydrate formation and blockage for nearly a century.” He adds that the issue is becoming more significant as drilling progresses into ever-deeper water and says the work by Varanasi’s team “is a big step forward toward finding more environmentally friendly ways to prevent hydrate obstruction in pipes.” The research team included MIT postdoc Adam Meuler and undergraduate Harrison Bralower; professor of mechanical engineering Gareth McKinley; St. Laurent Professor of Chemical Engineering Robert Cohen; and Siva Subramanian and Rama Venkatesan, two researchers from Chevron Energy Technology Company. The work was funded by the MIT Energy Initiative-Chevron program and Varanasi’s Doherty Chair in Ocean Utilization. A block of a gas hydrate (methane clathrate) recovered from seafloor sediments off the Oregon coast. Photo: Wusel007/wikipedia
越来越多的研究表明，水蒸汽（Water Vapour），而不是二氧化碳、甲烷和其他温室气体，是引起全球变暖的主要因素。 空气里的每4000个分子中只有1个是二氧化碳，而每20个分子中就有1个是水蒸汽。二氧化碳从太阳光中吸收的能量只是水蒸汽所吸收的量的1/4，因此大气变暖主要源于水蒸汽吸收的热量。水蒸汽是大气中数量最大的温室气体，根据一些估计，36%-85%的温室作用是水蒸汽引起的。而人类活动对其数量的直接影响很小。这一结论似乎说明了全球变暖是自然因素的后果，而非人类行为造成的。但是就此认为人类燃烧化石燃料，排放温室气体并未引起气候变化就确大错特错。因为人类行为的虽然对水蒸汽产生的直接影响很小，但是能够产生间接地、实质性地影响。二氧化碳、甲烷等温室气体放大了水蒸汽的加热作用，这一过程被科学家称为水蒸汽反馈（Water Vapour Feedback），这一正反馈循环的运行机制是怎样的：二氧化碳、甲烷和其他温室气体提供了原始加热动力，这会增加地面温度，地表温度提高加剧海洋蒸发，这将增加水蒸汽数量或空气湿度。而水蒸汽作为主要的温室气体将吸收大量更多的来自地球的热能。（温室气体使得短波太阳辐射穿过大气层，同时阻止地球表面的长波辐射逃离大气层。这一过程能够是地球的温度维持在一个适宜人类居住的水平。如果大气层中没有水蒸汽、二氧化碳、甲烷和其他温室气体，地球的气温应该在华氏-9度至-34度，而不是现在的比较适宜的59度左右。）水蒸汽反馈大概会使得加热作用增强一倍左右。 全球气温上升的主要原因就在于二氧化碳、甲烷等温室气体排放所间接引起的水蒸汽反馈作用的增强，而不是直接源于这些温室气体排放本身。因此认为水蒸汽是最重要的温室气体是正确，但认为二氧化碳不重要则是错误的。 相关文章链接： Andrew Dessler says water vapors role in warming now understood Water vapor and global warming Global Warming Supercharged by Water Vapor? The role of stratospheric water vapor in global warming Stratospheric Water Vapor Is a Global Warming Wild Card Water vapour caused one-third of global warming in 1990s Water-vapor feedback is strong and positive An Explanation for the Decade-Long Pause in Global Warming? Water Vapor Feedback Loop Will Cause Accelerated Global Warming Why has global warming paused? Water vapor may be in the answer 水蒸气是全球气候变化和全球变暖主要影响因素 平流层水蒸气浓度下降为全球变暖减速
标签: 甲烷
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