NMR investigation of Fischer-Tropsch waxes

Abstract

An investigation of the proton second moment and nuclear spin-lattice relaxation time as a function of temperature in two Fischer-Tropsch waxes having average carbon numbers of 28 and 28.5 is reported. The temperature dependence of the motional behaviour of the chains in the waxes differs significantly from the behaviour in the normal compound n-C₂₈H₅₈. As revealed by the second moment measurements, the rotational-diffusional (ROD) motions of the chains in the waxes set in at temperatures much below the transition to the hexagonal phase. The results are consistent with a model in which the chains of unequal length in the crystalline zones of the waxes are packed in an irregular, staggered arrangement. The onset of the ROD motions in the waxes is preceded by structural phase transitions which occur at temperatures depending on the range of carbon numbers in each wax. Activation energies for the different molecular motions are reported and compared with corresponding values in n-C₂₈H₅₈.