Estimation of Hydrogen Bond Distribution in Coal through the Analysis of OH Stretching Bands in Diffuse Reflectance Infrared Spectrum Measured by in-Situ Technique

Abstract

A new method was presented to estimate the strength distribution of hydrogen bonds in coal. The hydrogen bonds include the coal intramolecule hydrogen bonds and coal−water hydrogen bonds formed by hydroxyls in coal. The method analyzes the FTIR spectrum ranging from 2400 to 3700 cm-1 obtained using the in-situ diffuse reflectance IR Fourier transform (DRIFT) technique with neat, undiluted, coal samples. The FTIR spectra during the heat-up of eight coals (seven Argonne premium coals and an Australian brown coal), an ion-exchange resin, and a lignin were measured every 20 °C from room temperature to 300 °C. Each spectrum was divided into six hydrogen-bonded absorption bands by a curve-resolving method, then the amount of hydroxyls contributing to each hydrogen bond was estimated by Beer's law by using different absorptivity for each band. The strength of each hydrogen bond was estimated using a relation presented by Drago et al. that is known as one of the “linear enthalpy−spectroscopic shift relations”. Using this analysis method, changes in hydrogen bond distributions (HBD) with increasing temperature were successfully estimated for all the samples examined. By utilizing the HBD the changes in enthalpies associated with the desorption of adsorbed water, the glass transition, and the decomposition of COOH groups were well estimated. Only FTIR spectra measurements were found to be enough to obtain such enthalpies. This greatly simplified the calculation procedure and increased the accuracy of the enthalpies. The validity of the proposed in situ FTIR measurement method and the analysis method for obtaining HBD was well clarified.