Variable-Temperature High-Resolution Proton NMR Study of Laboratory-Frame and Rotating-Frame Spin−Lattice Relaxation in Coals

Abstract

We have carried out the first systematic in situ variable-temperature (25−180 °C) high-resolution proton NMR study of laboratory-frame and rotating-frame proton spin−lattice relaxation of coal samples, based on the CRAMPS technique. For coal samples that have been exposed to air, we confirmed the fact that paramagnetic oxygen is the main source of laboratory-frame proton spin−lattice relaxation (T 1). We demonstrate that paramagnetic oxygen trapped in coal can be used as a sensitive probe for monitoring structural and dynamical changes in coal as the temperature is varied. High-temperature spin−lattice relaxation experiments help to reveal the structural heterogeneity of coal because of reduced proton and electron spin-diffusion rates at high temperature. Large domains, on the order of 200−800 Å, with distinctively different paramagnetic oxygen concentrations, were found in all three coal samples studied, consisting of one low-volatile and two high-volatile bituminous coals from the Argonne Premium Coal bank. In particular, we found that aliphatic-rich domains with a length-scale larger than 500 Å exist in Premium Coal 601. The observed dependences of the rotating-frame 1H spin−lattice relaxation time T 1 ρ on the strength of the spin-lock field and temperature support the view that the main relaxation mechanism is time-dependent 1H−1H dipolar interactions in coals. From these dependences, we estimate that the correlation time of molecular motion responsible for rotating-frame proton spin−lattice relaxation in coals is on the order of 5 μs, which is in agreement with conclusions drawn from previous proton dipolar-dephasing studies. Two T 1 ρ values were identified for each of the three coal samples studied, indicating the existence of structural heterogeneity in coal on a spatial scale of at least 50 Å. The sizes of heterogeneous domains in coal are estimated on the basis of measured spin−lattice relaxation times and the analysis of proton spin-diffusion processes.

Determination of T1ρH Relaxation Rates in Charred and Uncharred Wood and Consequences for NMR Quantitation

“…Anomalously high T 1r H values reported for aromatic coal carbons [15,54] are most likely incorrect, due to the interference of slowly cross-polarizing 13 C nuclei in the VCT experiment. Whereas T 1r H is apparently longer for aromatic than aliphatic carbons in these VCT studies, the opposite is true when T 1r H is measured directly via 1 H detection [61]. The apparent lack of correlation of T 1r H with free radical content [15] is likely to result from the biases of the VCT experiment.…”

“…2b) [6-9, 25-28, 30, 31, 34, 43, 47, 54-59] and (iii) direct measurement through the 1 H channel (Fig. 2c) [6,9,26,51,56,[60][61][62][63]. Direct measurement offers the advantage of the high sensitivity and isotopic abundance of the 1 H nucleus and the ability to reliably detect components with very short T 1r H values (say 50:1 ms) [6,62,63].…”

“…Direct measurement offers the advantage of the high sensitivity and isotopic abundance of the 1 H nucleus and the ability to reliably detect components with very short T 1r H values (say 50:1 ms) [6,62,63]. However, the poor spectral resolution of solid-state 1 H NMR spectra is a major disadvantage, unless CRAMPS is used [61]. Also, the direct method detects inorganic 1 H pools (e.g.…”

CP dynamics of heterogeneous organic material: characterization of molecular domains in coals

“…The function decrease is slightly modulated, caused by the interplay of experimental features and sample properties (e.g. energy exchange between the rf-field, 1 H-1 H dipolar fluctuations, motional frequencies in the order of the rf-field, dipolar interactions in the order of the MAS frequency) [37].…”

“…In this context, the additional interpretation of T 1rH and T 1rC may help to assign physical and structural properties of this coal. Competing relaxation mechanisms may complicate a proper assignment in terms of mobility and modification [18,37]. A comparison of T 1rH for alkyl (T 1rH ¼ 1:1 ms) and aryl (T 1rH ¼ 8:2 ms) carbons within the crystalline domain of C1 indicates limited spin diffusion, explainable by differing mobility.…”