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要旨:
Skeletal reactions of n-hexane were studied over Pt/SiO2 (EuroPt-1) at conversions approaching zero. Various catalyst activation procedures involve the removal of different amounts of surface carbon as checked by XPS of carbon, therefore various activities are observed in the first runs. Activity vs, time as well as selectivity vs. conversion curves and optimum performance envelopes (OPE), i.e. yield vs. conversion plots are presented over catalysts cleaned by several oxidation-reduction cycles. In the very first moments of contact of n-hexane with the catalyst, on a relatively clean, hydrogen covered platinum (“Pt-H”), hydrogenolysis to methane and benzene formation are preferred reactions. A primary C5-cyclization, in some cases also isomerization is possible. Thereafter, a “Pt-C-H” system is built up gradually: single, internal hydrogenolysis and nondegradative reactions are characteristic of this period of catalyst life. Over a more severely carbonized catalysts (a “Pt-C” state), sequential terminal fission and alkene formation predominates. Aromatization seems to occur with rather constant selectivity over Pt-C-H and Pt-C. The lifetime of the Pt-H state is up to ca. 1-2 min at 603 K whereas Pt-C-H lasts for several minutes; higher hydrogen pressures preserve these states longer. At 693 K, the initial clean Pt (or Pt-H) can transform directly to Pt-C.
