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Reports: ND251786-ND2: Site-Specific Isotope Fractionation of Hydrocarbons by Quantitative NMR Spectroscopy

Juske Horita, PhD, Texas Tech University

Overview Bulk and compound-specific isotope analysis, and empirical and theoretical kinetic models of stable isotopes have made significant contributions to our knowledge on the sources and alteration pathways of oil and gases. However, their applications are still limited due largely to post-generative processes and a lack of suitable calibration data for kinetic modeling. Many organic molecules contain hydrogen and carbon in energetically nonequivalent positions (e.g., methyl and methylene groups of propane), and their isotopic composition can differ due either to equilibrium or to path-dependent kinetic processes. This site-specific or intramolecular 2H/1H and 13C/12C isotope fractionation between different H and C positions within a given hydrocarbon molecule (isotopomers) can provide additional information and much deeper insights into their sources and alteration pathways of petroleum hydrocarbons within source and reservoir rocks, including temperatures, conditions, and pathways of thermal cracking and other formation processes. In this proof of principle study, we propose to demonstrate that site-specific 2H/1H and 13C/12C isotope fractionations can be determined with sufficient precision for select light hydrocarbons and related petroleum products by means of quantitative 2H and 13C NMR spectroscopy.   Analytical Methods The focus of the research is on the light alkanes, the major components of petroleum fluids, including propane, butane, pentane, and cyclohexene.  1H, 2H, and 13C NMR spectra were recorded on a JEOL SCC-400MHz spectrometer operating at 399.7822 MHz (1H), 61.369 MHz (2H), 100.5253 MHz (13C).  The instrument was automatically tuned for the appropriate nucleus prior to analysis.  Samples of neat liquid were run without lock in a Wilmad 5 mm Step-down Ultra-Thin Wall Precision NMR Sample Tube.  Proton decoupling was used to prevent splitting of the resonances and maximize signal-to-noise.  Acquisition time was sufficiently long to allow for the full decay of the FID, and a relaxation delay was greater than or equal to five time T1 allowing for the nucleus to fully relax.  The number of scans was chosen such that the smallest resonance had a S/N of 150 or greater.  The S/N was improved with the line broadening factor, sexp = 0.2.  The digital resolution was increased by zero-filling 4 times.  Phasing was performed automatically but manually refined.  The resolution, checked by measuring the peak width at half height without line-broadening, was less than 0.5 Hz.   Results and Discussion   2H NMR: The deuterium NMR spectra for pentane (Figure 1) and cyclohexene have been previously reported. Pentane show the two methylene and one methyl functionalities of 4.000 : 2.0126 : 5.7163 deviating from the expected statistical distribution, 4 : 2 : 6.  The deuterium NMR spectrum of cyclohexene also shows a fractionation of 2 : 3.852 : 3.8318 between the different functionalities, one alkene and two alkane.  This differs from the statistical distribution of 2 : 4 : 4, shows an increase in the alkene fractionation by 38‰.  

http://prf.confex.com/data/abstract/prf/2013/Paper_12342_abstract_20263_0.jpg Figure 1.  2H NMR spectrum of neat pentane.

  Butane and propane are gases at room temperature and pressure, and presents a special challenge.  To be analyzed by NMR spectroscopy using methods currently available to us, the gas must be condensed.  To allow analysis at ambient temperature, an NMR tube must tolerate the higher pressures associated with maintaining liquid at ambient temperature.  A regular thickness Wilmad J-Young style NMR tube has a tolerance of approximately 10 bar, which is sufficient to hold butane with a safety margin, which has about 3 bar vapor pressure at room temperature. The deuterium NMR spectrum for butane (Figure 2) shows two separable resonances A and B with an expected statistical distribution of 4 : 6. The observed ratio, 4.000 : 5.9661 shows a much smaller degree of fractionation.

Figure 2.  2H NMR spectrum of butane.   Due to the lower molecular weight and resulting higher pressure of propane, a customized NMR tube with valve assembly is required.  A hollow tube of machined sapphire sealed on one end is the recommended material for elevated pressure NMR spectroscopy, and titanium was chosen for the valve because of its non-magnetic and relative lightweight properties.  The design of the valve allows for mounting the assembly to a vacuum manifold and easily admitting neat propane to be condensed cryogenically, and easily venting when analysis is complete (Figure 3).  Although titanium is a lightweight material, it is still too heavy to allow spinning the sample which makes gradient shimming somewhat difficult on current equipment.    Figure 3. Custom-made sapphire NMR cell and Ti valve assembly. Note liquefied propane within the cell.  

Figure 4.  Preliminary 2H NMR spectrum of propane.   13C NMR: Quantitative carbon-13 NMR spectroscopy is used less often for site-specific isotope fraction determination due to some unique challenges.  The low sensitivity, low abundance, and the long relaxation time of carbon make for especially long analysis times.  Our preliminary testing shows that this can be mitigated by using a low concentration of the relaxation agent, Cr(acac)3 and very high concentrations of analyte. Another, possibly more significant challenge, is peak enhancement by NOE generated by bonded protons during decoupling.  This can be minimized by proton decoupling during only a minimal acquisition period.  In order to achieve 1‰ (0.1%) accuracy, the decoupler must also be fine tuned.  A series of experiments was performed on a sample of 13CH313CH2OH by positioning the decoupler offset on varying positions of the ethanol spectrum.  When the integration of both 13C peaks are identical, the decoupler offset is appropriately set.  On the new JEOL 400 spectrometers, the impact of varying the decoupler offset was minimal.   Conclusion and Future Work   Deuterium analysis of the some small hydrocarbons, butane, pentane and cyclohexene, demonstrate the feasibility of site-specific 2H/1H isotope fractionation, which corresponds to the type of chemical environment. Work is ongoing to determine the fractionation in propane, using the custom-developed sapphire cell with titanium valve assembly. Quantitative carbon-13 NMR spectroscopy will be further refined and developed for light hydrocarbons (propane, butane, pentane).   Funding from NSF grant CHE-1048553 and the NSF's CRIF program which supported the purchase of this instrument, is gratefully acknowledged.