1H Diffusion-Ordered Spectroscopy (DOSY) Nuclear Magnetic Resonance (NMR) as a Powerful Tool for the Analysis of Hydrocarbon Mixtures and Asphaltenes

Abstract

Heavy crude oils are more and more of interest for the oil industry to meet the growing worldwide energy demand. Asphaltenes, which are the heaviest and least reactive molecules in crude oils, have received great attention over the last few decades, because they are responsible for many problems occurring during hydrotreatment and hydroconversion. 1H diffusion-ordered spectroscopy (DOSY) nuclear magnetic resonance (NMR) based on pulsed field gradient (PFG) sequences is a powerful tool to analyze polydisperse samples. The key advantage of such a technique compared to traditional NMR diffusion sequences, such as pulsed field gradient spin−echo (PFGSE), is to obtain both physical and chemical information in a single experiment. 1H DOSY NMR has been carried out on different types of samples to obtain a deeper insight into the physicochemical properties of petroleum samples, because diffusion coefficients are both sensitive to molecular weights and sizes. The application of DOSY NMR to analyze hydrocarbon mixtures and asphaltenes is assessed. It is shown that both solute and solvent diffusion coefficients decrease with an increasing concentration of solute. Different types of intermolecular interactions were observed on petroleum samples depending upon the sample concentration. Dilute and semi-dilute regimes have also been detected. 1H DOSY spectra applied to diesel and asphaltene samples in toluene are presented and interpreted to demonstrate the potential of DOSY techniques to analyze heterogeneous petroleum samples. The data obtained for the diesel sample enable us to establish that monoaromatics were connected to long alkyl chains, whereas di- or triaromatic molecules were linked to shorter hydrocarbon chains. However, a clear conclusion could not be reached for the asphaltene sample because there were only a few aromatic protons that were not detected. This observation is most consistent with a continental model of these asphaltenes.