Measurement of -OH groups in coals of different rank using microwave methodology, and the development of quantitative solid state n.m.r. methods for in situ analysis

Abstract

Experiments with both model compounds (substituted phenols) and with 11 coals (nine British and two American) have established that microwave heating will greatly accelerate silylation reactions of the phenolic -OH groups, e.g. for Creswell coal complete silylation of -OH groups occurs in 35 min in the microwave oven, whereas 24 h is required using a bench reflux technique. Microwave reaction times for coals vary from 35 min to 3 h for more dense coals such as Cortonwood. The above observations have allowed the development of a ‘one pot’ silylation of coal, followed by an in situ analysis of the added Me 3Si- groups by quantitative 29Si magic angle spinning nuclear magnetic resonance (MAS n.m.r.) spectroscopy. The development of a quantitative n.m.r. method required the determination of 29Si spin lattice relaxation times, T 1, e.g. for silylated coals T 1 ∼ 8 s; for silylated phenols, T 1 ∼ 25 s; for the synthetic smectite clay laponite, T 1 ∼ 25 s; and for Ph 3SiH, T 1 ∼ 64 s. Inert laponite was selected as the standard. The requirement to wait for five T 1 max between pulses, together with the relatively low natural abundance of 29Si (4.71%), results in rather long accumulation times to obtain spectra of analytical quality (8–48 h). However, in comparison with other methods, even in the most unfavourable case, the total time from commencement of analysis to result may be described as ‘rapid’. The results for O OH O total obtained are compared with other literature data. Comparison with ketene data, for example, shows agreement to vary from excellent (Creswell) through satisfactory (Cortonwood) to poor (Pittsburgh). Even in cases where agreement with ketene data is less good, the silylation result may be close to estimates made via other acetylation methods. Possible reasons for the variations observed are discussed.