Experimental constraints on the equilibrium hydrogen isotopic composition and rates of hydrogen isotopic exchange of C-H bonds in natural gas

天然气是美国经济的关键组成部分。甲烷也是一种温室气体，是生命的产物和食物来源。追踪甲烷源和汇被用于能源勘探、气候科学和生物科学。发现天然气来源的主要方法之一是测量碳和氢的稳定同位素值，但使用同位素比率来识别天然气来源要求这些比率在天然气在地球上积累和储存期间不发生变化。但这是否属实并不为人所知。拟议的工作将在实验室条件下测量氢同位素交换速率和最终平衡值。这将被用来评估地球上氢同位素比率可能发生变化的条件。这些信息将对包括地球科学家、能源公司和致力于识别天然气来源的大气科学家在内的各种团体有用。随着美国继续从页岩中提取天然气并出口，这项工作只会变得更加适用。该项目将培养一名研究生，并将通过加州大学伯克利分校现有的项目为本科生提供研究机会，并将在一年一度的加州大学日活动中与社区分享校园研究。使用天然气氢同位素组成作为天然气来源指纹的关键是必须假设样品的同位素组成在形成后不会改变。这可能是由于在天然气形成之后但在取样之前地下发生的氢同位素交换反应造成的。有问题的是，天然气在地球近地表(200摄氏度)相对较低的温度下进行氢同位素交换反应的条件限制很差。为了能够评估这种交换反应是否确实在自然界中发生并影响天然气的氢同位素组成，需要：(1)了解这些气体在相关自然条件下与水等常见含氢分子的氢同位素交换速率；(2)烷烃和潜在的氢同位素交换伙伴之间的平衡同位素组成，其值决定进行氢同位素交换的天然气的最终同位素组成。到目前为止，对于低温(200摄氏度)的烷烃气体，氢同位素交换速率和预期的平衡组成都鲜为人知。但要完全理解在低温环境下测得的天然气的氢同位素组成反映了源区和后地层的历史，就必须了解这些知识。为此，将进行一系列实验，旨在通过实验确定：(1)25-250℃范围内，包括甲烷簇状同位素在内的C_1-C_4烷烃气体氢同位素分布的预期平衡依赖关系，以及(2)C_1-C_5烷烃与水在多种温度下的交换速率，以使地质相关系统的交换速率可以外推到低温。将用于完成这些任务的方法是：(1)使用催化剂来平衡碳氢化合物和氢气；(2)在金电池反应堆中，在高温和压力下将气体与水加热；以及(3)测量所有实验中碳氢化合物的同位素组成。这项工作将对与水发生同位素交换反应的天然气的氢同位素组成以及气体达到氢同位素平衡后的同位素组成提供新的限制。该奖项反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

Natural gas is a key part of the US's economy. Methane is also a greenhouse gas and product and food source for life. Tracking methane sources and sinks is used in energy exploration, climate science, and biosciences. One of the main ways that natural gas sources are found is by measurement of carbon and hydrogen stable isotope values but use of isotope ratios to identify gas sources requires that those ratios do not change during gas accumulation and storage in the Earth. But whether this is true is not well known. The proposed work will measure rates of hydrogen isotope exchange and the final equilibrium values under laboratory conditions. This will be used to evaluate conditions where hydrogen isotope ratios could change in the earth. This information will be useful to a variety of groups including earth scientists, energy companies, and atmospheric scientists working to identify natural gas sources. This work will only become more broadly applicable as the US continues to extract natural gas from shale and exports it. The project will train one graduate student and will offer research opportunities to undergraduate students through established programs at UC Berkeley, and the will be highlighted at the annual "Cal" Day event where research on campus is shared with the community.Critical to the use of natural gas hydrogen isotopic compositions as a fingerprint for gas origins is the necessary assumption that isotopic compositions of samples are not modified post formation. This could occur due to hydrogen isotope-exchange reactions occurring in the subsurface after gas formation but before sampling. Problematically, the conditions under which natural gas undergoes hydrogen isotope exchange reactions at relatively low temperatures in Earth's near surface (200oC) is poorly constrained. To be able to evaluate whether such exchange reactions actually occur in nature and influence natural gas hydrogen isotopic compositions requires: (i) Knowledge of the rates of hydrogen isotope exchange of these gases with common hydrogen bearing molecules such as water at relevant natural conditions; and (ii) The equilibrium isotopic composition between alkanes and potential hydrogen-isotope exchange partners, the value of which determines the final isotopic composition of the natural gases undergoing hydrogen isotope exchange. To date, both the rates of hydrogen isotope exchange and the expected equilibrium compositions are poorly known for alkane gases at low temperatures (200oC). But knowledge of such are necessary for a complete understanding of what measured hydrogen isotopic compositions of natural gas in low temperature environments reflect about source vs. post-formational histories. To this end, a series of experiments will be conducted that are designed to establish experimentally: (i) the expected equilibrium dependence of hydrogen isotopic distributions for C1-C4 alkane gases from 25-250oC, including for methane clumped isotopes and (ii) the rates of exchange of C1-C5 alkanes with water at multiple temperatures such that the rates of exchange can be extrapolated to low temperatures for geologically relevant systems. The methodologies that will be used to accomplish these tasks are: (1) The use of catalysts to equilibrate hydrocarbons with H2 gas; (2) Heating of gases with water in gold cell reactors at elevated temperature and pressures; And (3) measurement of the isotopic composition of the hydrocarbons from all experiments. This work will provide the new constraints on what sets the hydrogen isotopic composition of natural gas undergoing isotope exchange reactions with water as well as what the isotopic compositions will be once the gases are in hydrogen isotopic equilibrium.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.