Composition of carbonaceous deposits and catalyst deactivation in the early stages of the hydrocracking of a coal extract

Carbonaceous deposits on finely dispersed MoS2 particulates and a NiMo-supported catalyst, recovered from coal extract hydrocracking experiments, have been examined. Large surges in carbon deposition on catalyst surfaces were observed when using fresh catalyst and fresh feed. When using fresh feed with re-used catalyst, additional but smaller surges in deposition were found. The compositions of the deposits appear to reflect, at least in part, the composition of the coal liquefaction extract. The relative stability in the amounts of carbonaceous deposit at reaction times longer than 2 h miners earlier findings suggesting that deactivation of the catalyst after the first 2 h of usage (up to 6 h) was marginal. Most of the deposition of carbonaceous material was found to precede completion of the activation (by sulphidation) of the catalysts. Moreover, much greater loadings of deposits were found on the finely dispersed sulphided-Mo catalyst compared with the supported NiMo/gamma-Al2O3. Nevertheless, broadly comparable conversions of + 450 degrees C boiling fractions have earlier been observed in the presence of the two catalysts, at reaction times longer than 30 min. Both these catalysts appear to function at their observed levels of activity, whilst carrying high levels of carbonaceous deposits. These findings tend to confirm earlier work (Thompson SJ, Webb G. Journal of the Chemical Society, Chemical Communications 1976:526; Djega-Mariadassou, pers comm) suggesting that carbonaceous deposits might act as a permeable layer, and perhaps act as a reservoir of hydrogen - shuttling hydrogen from the liquid phase to molecules on the catalyst surface. (C) 1999 Elsevier Science Ltd. All rights reserved.