Hydrogen isotope exchange in thermally stressed cellulose

We have investigated the changes in the ratio of stable isotopes of hydrogen when cellulose is subjected to thermal stress at the high temperatures of 180°C and 200°C, respectively. The thermal stress caused deuterium depletion at both the temperatures, with the kinetics being faster at the higher temperature. The process appeared to approach isotopic equilibrium when the D/H of heated cellulose samples extracted from several wood samples were compared with their non-exchangeable D/H. Previous work has shown that the cellulose polymer contains exchangeable hydrogens bonded as O–H species and that this hydrogen is preferentially substituted by deuterium when cellulose is allowed to exchange in aqueous solutions at 0°C. Thermal stress apparently involves the reverse reaction i.e. preferential incorporation of hydrogen from the exchanging medium. Because of the high experimental temperatures involved, it is likely that the exchange took place between the O–H hydrogens in cellulose and a vapor phase. Further, the thermal stress also resulted in varying degrees of hydrogen loss from the cellulose molecule. This study suggests that by using a pre-heating technique, it might be possible to determine the D/H ratio of non-exchangeable hydrogens in a variety of samples. The technique is also useful when determining the stable hydrogen isotope ratio of kerogen extracted from sediments since it is often difficult to determine and eliminate all, if any, of the exchangeable hydrogen in these types of samples.