13C NMR Derived Average Molecules - Heavy Petroleum - ISMAR 2013


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13C NMR Derived Average Molecules - Heavy Petroleum - ISMAR 2013

  1. 1. Calculation of average molecular descriptions of heavy petroleum hydrocarbons by combined analysis by quantitative 13C and DEPT-45 NMR experiments John C. Edwards*, A. Ballard Andrews† * Process NMR Associates, LLC, 87A Sand Pit Rd, Danbury, CT 06810 USA (*e-mail: john@process-nmr.com) † Schlumberger-Doll Research, Cambridge, MA, USA Overview Much debate has centered around the validity and accuracy of NMR measurements to accurately describe the sample chemistry of heavy petroleum materials. Of particular issue has been the calculated size of aromatic ring systems that in general seem to be underestimated in size by NMR methods. This underestimation is principally caused by variance in chemical shift ranges used by researchers to define the aromatic carbon types observed in the 13C NMR spectrum, in particular the bridgehead aromatic carbons that can be shown to overlap strongly with the protonated aromatic carbons. The ability to discern between bridgehead aromatic carbons and protonated carbons in the 108-129.5 ppm region of the spectrum is key in the derivation of molecular parameters that describe the “molecular average” present in the sample. Utilizing methodologies developed by Pugmire and Solum for the solid-state 13C NMR analysis of coals and other carbonaceous solids [1], we have developed a new liquid-state 13C NMR method that allows the relative quantification of overlapping protonated and bridgehead aromatic carbon signals to be determined [2]. The NMR experiments involve the combined analysis of both quantitative 13C single pulse excitation which observes all carbons quantitatively, and a DEPT45 polarization transfer which observes only the protonated carbons in the sample. Though the DEPT45 results are not quantitative across all carbon types (CH, CH2, and CH3) due to polarization transfer differences, the technique is well enough understood that simple multiplication factors allow the relative intensities of the different carbons to be determined. The average ring system sizes derived from these NMR experiments tend to be several ring systems larger than has been calculated in previous studies [3]. In heavy petroleum asphaltenes the average aromatic ring system is 5-7 rings in size which is in agreement with FTICR-MS and fluorescence measurements [4-7], rather than the 3-4 rings previously reported [3]. The results obtained by liquid-state NMR are compared to the carbon-type NMR and molecular parameters derived from the solid-state 13C analysis of the same samples. A chemometric approach to the derivation of the parameters has been developed so that automated output of the analysis results can be routinely provided by technician level personnel. Finally, a quantitative 13C NMR method is described that utilizes internal standards to quantify the amount of carbon in the sample observed in the NMR experiment. This 13C quantitation yields information on the degree of aggregation in the asphaltene components. Experimental 13C NMR spectra were obtained on a Varian Mercury 300-MVX at a resonance frequency of 75.43 MHz. Single Pulse Excitation (SPE) experiments were performed with inverse gated 1H decoupling and with 0.025M CrAcAc present in the CDCl3 solvent to facilitate the accumulation of quantitative NMR data. Samples typically contain around 80-100 mg of sample dissolved in 1 gram of CDCl3-CrAcAc. DEPT45 experiments were obtained on the same sample. DEPT Experiment 13C NMR Analysis Examples 13C qNMR 13C DEPT-45 NMR Comparison of Quantitative NMR with DEPT-45 NMR Integration of respective aliphatic carbon zones for normalization calculation Carbon Type Multiplication Factor CH 1.414 CH2 1.000 CH3 0.943 Carbon Chemical and Molecular Parameters Relationship between mole fraction of bridgehead carbons χb and aromatic cluster size (C). From Solum, et al. Energy& Fuels, 3, 187, 1989. Aggregation estimation by qNMR - internal standard technique. PEG acts as a carbon content standard allowing observed carbon to be calculated as well as acting as the DEPT normalization standard. 1) Pugmire et al., Energy & Fuels, 3, 187, 1989. 2) Andrews, Edwards, et al., Energy & Fuels, 2011, 25 (7), pp 3068–3076 3) Sheremata et al., Energy& Fuels, 6, 414, 2004. 4) Badre et al., Fuel, 85, 1, 2006. 5) Sharmar et al, Energy & Fuels 16, 490, 2002. 6)Groenzin and Mullins, “Asphaltenes, Heavy Oils and Petroleomics, Ch. 2. 7) Rodgers and Marshall, “Asphaltenes, Heavy Oils and Petroleomics, Ch. 3. For more details see: " Comparison of Coal-Derived and Petroleum Asphaltenes by 13C Nuclear Magnetic Resonance, DEPT, and XRS", A. Ballard Andrews, John C. Edwards, Andrew E. Pomerantz, Oliver C. Mullins, Dennis Nordlund, and Koyo Norinaga, Energy Fuels, 2011, 25 (7), pp 3068–3076 13C SPE NMR S