Reactions of n-alkanes on and removal of carbonaceous residues from Pt catalysts Original Research Article

Pt-black, 6% Pt/SiO2 and a 0.6% industrial Pt/A12O3 catalysts were compared in reactions of n-nonane. The main products were olefins and fragments. Of the three samples, Pt/A12O3 showed the lowest degradative activity. Products of direct C6- and C5-cyclization were present, together with minor amount of trimethylbenzene. Isomers contained mostly monobranched ones which could have been formed by the C5-cyclic or the bond shift route. The fragment composition pointed to metal catalysed hydrogenolysis. The C6 and C7 fragments also contained aromatic and cyclopentanic products. Long-term runs led to consumption of primary olefins into fragments (Pt-black and Pt/SiO2) and also into aromatics on Pt/A12O3. n-Nonane was, as a rule, less reactive than n-hexane which latter produced more nondegradative products (isomers, C5-cyclics and benzene). The composition of hydrocarbons removed by hydrogen treatment after run (with ca. 10% conversion)was determined first of all by the nature of the catalyst. It contained much methane in the case of Pt-black whereas much benzene was removed from supported catalysts, both after n-hexane and n-nonane reaction. In the latter case n-nonane was the only C9 hydrocarbon in the removed fraction. Higher hydrogen excess during catalysis led, as a rule, to higher conversions but hardly affected the amount and composition of the removed hydrocarbons with one spectacular exception: methylcyclopentane left EUROPT-1 after a run of with much H2. We propose that the composition of removed hydrocarbons may reflect the structure of hydrocarbonaceous adspecies during reaction with supported catalysts while it may transform into carbonaceous deposits on Pt-black. H2 can split them up to C1 units, appearing as methane in the gas phase. Surface electron spectroscopy is in agreement with this latter assumption.