Solid-State NMR Studies of Fossil Fuels using One- and Two-Dimensional Methods at High Magnetic Field

Abstract

We examine the opportunities offered by advancements in solid-state NMR (SSNMR) methods, which increasingly rely on the use of high magnetic fields and fast magic angle spinning (MAS), in the studies of coals and other carbonaceous materials. The sensitivity of one-and two-dimensional experiments tested on several Argonne Premium coal samples is only slightly lower than that of traditional experiments performed at low magnetic fields in large MAS rotors, since higher receptivity per spin and the use of 1 H detection of low-gamma nuclei can make up for most of the signal loss due to the small rotor size. The advantages of modern SSNMR methodology in these studies include improved resolution, simplicity of pulse sequences, and the possibility of using J-coupling during mixing. ■ INTRODUCTIONThe ever increasing need to optimize conversion of heavy fossil fuel resources into useful products in an environmentally benign and cost-effective manner requires detailed understanding of the molecular structure and the reactivities. 1 One of the most powerful analytical methods for studying insoluble carbonaceous materials in bulk is solid-state nuclear magnetic resonance (SSNMR) spectroscopy, which for over three decades has been used as the primary source of information about concentrations of various carbon and hydrogen functionalities. 2−11 Numerous early investigations have suggested that the quantitative (to within a few %) 13 C intensities in coals could be best measured at low magnetic field, B 0 of 4.7 T or less, under slow magic angle spinning (MAS), at rates of 10 kHz or less, using variable-contact time cross-polarization (CP) or direct-polarization (DP) MAS experiments. 4−10 Specifically, it was accepted that the seemingly conflicting requirements of using MAS rates that exceed 13 C chemical shift anisotropies (CSAs), yet do not interfere with the CP process, could be best met under such conditions. Second, the inhomogeneously broadened lines in coals scale linearly with B 0 , which partly negates the resolution and sensitivity advantage of a higher field. Lastly, the high-resolution 1 H NMR studies of coals using combined rotation and multiple-pulse spectroscopy (CRAMPS) were also carried out under low-field/slow-MAS conditions. 8,12,13 The continuous development of stronger magnets, more sensitive probes, operating at higher MAS rates, innovative pulse sequences, and improved computational tools has led to dramatic progress in SSNMR spectroscopy. Advances in ultrafast MAS technology, 14 which allow for sample spinning at 40−80 kHz, opened new opportunities for refining multidimensional SSNMR experiments. 15−20 The impact of fast MAS relies on excellent sensitivity per spin, great flexibility in using the radiofrequency (RF) magnetic fields, efficient CP transfer, increased frequency range of the indirect dimension in rotor-synchronized experiments, and elimination of the spinning sidebands in the presence of large CSAs. In addition, fast MAS by itself or in combination with RF pulse sequences (CRAMPS) can be us...

Advanced solid-state NMR spectroscopy of natural organic matter

“…Note that NMR characterization of fossil fuels has almost exclusively been performed at low magnetic fields, at 9.4 T (corresponding to a 1 H resonance frequency of 400 MHz) or lower and at spinning rates ≤ 20 kHz, because of several potential drawbacks associated with using high fields and high MAS rates [93]. Recent work by Pruski's group [293,294], however, demonstrated that high magnetic fields and fast MAS utilized in coal studies offered some advantages (the possibility of using 1 H detection of 13 C in 2D heteronuclear correlation schemes and simplified pulse sequences), with only slightly lower sensitivity. However, it remains difficult to detect interior aromatic carbons or obtain quantitative 13 C spectra under these conditions.…”

Characterization of biomass and its derived char using13C-solid state nuclear magnetic resonance

“…We end this section by referring to a recent work by the group of Pruski [112] strengthening the fact that application of ssNMR to the study of coals has mainly been developed in the 80's and since then few innovations have been brought in this field despite the fact that the "NMR tool box" has considerably developed in the past 20 years in terms of accessibility to strong magnetic fields (> 9.4 T), fast MAS (> 30 kHz), newly implemented pulse programs, improved electronics and so forth. For this reason, many classical approaches (e.g., use of TOSS to suppress spinning side bands, use of low magnetic fields) should be revised.…”

“…This was done, for instance, by us in the study of hydrothermal carbons and detailed later. Reference [112] shows then how structural resolution of various types of coals can be achieved at high fields and fast MAS by proper implementation of CP using, for instance, a tangential ramp to be more efficient in meeting the Hartmann-Hahn condition; it also shows how 2D 1 H- 13 C correlation maps can be obtained in a reasonable amount of time and acquired under advanced homonuclear decoupling schemes like Phase Modulated Lee-Goldburg (PMLG).…”

Protonated Nitrogen Structure in 15N-Labeled Model Coal Investigated by Solid-State 1H–15N Double-CP NMR Experiments under Ultrafast Magic-Angle Spinning

“…In the UF-MAS technique, the sensitivity per unit volume is also enhanced by reducing the spectral line width and/or SSBs, and 1 H inverse detection enables obtaining more than 10 times larger spectral intensity than 15 N detection in the case of 1 H– 15 N interaction . However, 1 H– 15 N observation of a natural coal using UF-MAS is assumed to not be feasible because the sample volume for UF-MAS is at most 1 mg; even a 1 H inverse heteronuclear correlation experiment for 13 C in coals, which is the most abundant nucleus in coals and has higher gyromagnetic ratio than 15 N, needs at least 23–68 h, making it very challenging . Therefore, for the detailed assignment of 1 H– 15 N correlation signals of natural coals, 15 N-labeled model coals that have 1D 15 N SPE MAS line shapes comparable to those of natural coals would be needed.…”

“…11 However, 1 H− 15 N observation of a natural coal using UF-MAS is assumed to not be feasible because the sample volume for UF-MAS is at most 1 mg; even a 1 H inverse heteronuclear correlation experiment for 13 C in coals, which is the most abundant nucleus in coals and has higher gyromagnetic ratio than 15 N, needs at least 23−68 h, making it very challenging. 12 Therefore, for the detailed assignment of 1 H− 15 N correlation signals of natural coals, 15 N-labeled model coals that have 1D 15 N SPE MAS line shapes comparable to those of natural coals would be needed. Details of the nitrogen structure for natural coals should then be studied based on such 15 N-labeled model coals.…”