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