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

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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.