Oxygen-free conversion of methane to higher hydrocarbons through a dual-temperature two-step reaction sequence on platinum and ruthenium - 2. Removal of products at a fixed temperature

In order to study the effect of the temperature (T-CH4) at which the catalysts were exposed to CH4 upon the C2+ products of the further hydrogenation, the latter process was conducted at a fixed temperature (T-H2) while different values were ascribed to T-CH4 ina set of successive experiments. T-CH4 did not exceed 320 degrees C. Different sets were effected, corresponding to different values of T-H2 (from room temperature to 200 degrees C). If T-H2 less than or equal to 120 degrees C hydrogenolysis was negligible, so that the variations of the production were only due to the changes affecting the adlayer. On both catalysts and at every value of T-H2, the production displayed a maximum versus TCH4 On Pt, the selectivity to ethane and propane at a given T-H2 monotonically decreased when T-CH4 increased while that corresponding to the heavier products (C-6-C-8) increased. In contrast, on Ru, the selectivities to ethane and propane exhibited a minimum versus T-CH4 while those to heptanes and octanes exhibited a maximum. The results are interpreted by assuming that, during the chemisorption of CH4, several families of hydrocarbon precursors of different weights and shapes are formed on the metal surfaces. It must be assumed that, on Pt, the average weight of the precursors was an increasing function of T-CH4, whereas on Ru it displayed an optimum in the upper part of the explored range of T-CH4. No appreciable amount of unreactive carbon was formed under the conditions of the described experiments. (C) 1998 Academic Press.