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Solid State and Solution State NMR Analyses of Ecuadorian Asphaltenes: Quantitative Solid State Aromaticity Determination Supporting the “Island” Structural Model. Aliphatic Structural Information from Solution State 1H-13C HSQC Experiments. Lawrence B. Alemany, Manjusha Verma, W. E. Billups, Scott L. Wellington, and Michael Shammai Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b01455 • Publication Date (Web): 03 Sep 2015 Downloaded from http://pubs.acs.org on September 13, 2015
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Solid State and Solution State NMR Analyses of Ecuadorian Asphaltenes: Quantitative Solid State Aromaticity Determination Supporting the “Island” Structural Model. Aliphatic Structural Information from Solution State 1H-13C HSQC Experiments.
Lawrence B. Alemany,† Manjusha Verma,‡ W. E. Billups,*,‡ Scott L. Wellington,§ and Michael Shammai¥ †Department of Chemistry, Shared Equipment Authority, and Smalley-Curl Institute, Rice University, 6100 Main Street, Houston, Texas 77005, United States
‡Department of Chemistry and Smalley-Curl Institute, Rice University, 6100 Main Street, Houston, Texas 77005, United States § Department of Chemical and Biomolecular Engineering, Rice University, 6100 Main Street, Houston, Texas 77005, United States ¥ Baker Hughes|Heavy Oil Research, 5555 San Felipe Street, Houston Texas, 77056, United States
Abstract: Solid state and solution state NMR studies of six Ecuadorian asphaltenes have provided insight into using NMR to characterize asphaltenes.
For these asphaltenes, free radicals prevent
obtaining quantitatively meaningful 1H-13C CPMAS spectra but facilitate obtaining meaningful carbon aromaticity values in direct polarization with
13
C pulse excitation.
The benefit of studying any
asphaltene by obtaining a series of CPMAS spectra with multiple contact times—rather than just a single contact time—is demonstrated. Analyzing the CPMAS and direct 13C pulse spectra indicates that, on average, condensed aromatic ring systems become larger as the aromaticity increases, which seems more consistent with “island” than with “archipelago” aromatic structures, just as recently proposed for asphaltenes. Solution state 2D 1H-13C HSQC spectra of the asphaltenes provide a wealth of structural information and are particularly useful for demonstrating that isolated methyl branches on alkyl chains are much more abundant than aromatic methyl groups. The prospect of using cryoprobes to obtain 1D and 2D spectra with much higher S/N allowing a more detailed analysis of the less abundant structural environments is clearly attractive. 1. Introduction
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Despite its importance in modern life, petroleum remains a complex and poorly understood material.1 For example, the efficient recovery and upgrading of oil from many reservoirs continues to be challenging. A major impediment for the upgrading of oil from many reservoirs is the absence of a protocol for the handling of asphaltenes, a major constituent of heavy oil. The asphaltenes are composed of a complex mixture of polycyclic aromatic hydrocarbons that are substituted by short alkyl groups and contain heteroatoms, including nitrogen, sulfur, oxygen, nickel, vanadium and other metals. Although mass spectrometry2-4 has played a critical role in understanding the properties of these complex materials, the molecular structures are still poorly understood. They are defined by their insolubility in n-pentane or n-heptane and their solubility in benzene and toluene. A salient feature of crude oil that contains asphaltenes is an enhancement of viscosity that arises from the formation of aggregates that are believed to be formed as a result of the attractive forces between the aromatic cores. Petroleum- and coal-derived asphaltenes have been studied extensively by solution state and solid state NMR techniques in an attempt to gain insight into the structure of asphaltenes. The data show that free radicals in the Ecuadorian asphaltenes studied significantly affect critical solid state NMR relaxation times. Specifically, the 1H spin-lattice relaxation time in the rotating frame T1ρ is not long enough for any of the samples to allow quantitatively meaningful CPMAS spectra to be obtained; however, the free radicals are apparently sufficiently shortening all the 13C spin-lattice relaxation times so that meaningful aromaticity values can be obtained in a direct
13
C pulse experiment with a 180s relaxation delay. The
aromaticities so obtained range from 38% to 52%. Analyzing cross polarization dynamics and relative 13
C spin-lattice relaxation times in the direct 13C pulse experiments on the various asphaltenes indicates
that, on average, condensed aromatic ring systems become larger as the aromaticity increases. This seems more consistent with “island” than with “archipelago” aromatic structures, as recently proposed for asphaltenes.5-10 2. Background 1
H-13C cross polarization (CP) with magic angle spinning (MAS) is a workhorse technique in solid
state NMR studies. Some CPMAS studies of asphaltenes used a single contact time (tcp, often about 1 2 ACS Paragon Plus Environment
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ms).11-20 One study included the effect of varying the relaxation delay.13 Other CPMAS studies of asphaltenes utilized multiple tcp values to study cross polarization dynamics and the effect of tcp on the observed carbon aromaticity.13,20-26 Depending on the study, tcp values ranged from 10 µs to 6 ms. Obtaining a meaningful carbon aromaticity for a given asphaltene requires that all of the carbons be uniformly cross polarized. As a practical matter, this requires that TCH
