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

Pt-black, 6% Pt/SiO2 and a 0.6% industrial Pt/Al2O3 catalysts were compared in reactions of n-nonane. The main products were olefins and fragments. Of the three samples, Pt/Al2O3 showed the lowest degradative activity. Produc ts of direct C-6- and C-5-cyclization were present, together with minor amo unt of trimethylbenzene. Isomers contained mostly monobranched ones which c ould have been formed by the C-5-cyclic or the bond shift route. The fragme nt composition pointed to metal catalysed hydrogenolysis. The C-6 and C-7 f ragments 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/Al2O3. n-Nonane was, as a rule, less reacti ve than n-hexane which latter produced more nondegradative products (isomer s, C-5-cyclics and benzene). The composition of hydrocarbons removed by hyd rogen 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 a fter n-hexane and n-nonane reaction. In the latter case n-nonane was the on ly C-9 hydrocarbon in the removed fraction. Higher hydrogen excess during c atalysis led, as a rule, to higher conversions but hardly affected the amou nt and composition of the removed hydrocarbons with one spectacular excepti on: methylcyclopentane left EUROPT-1 after a run of with much H-2. We propo se that the composition of removed hydrocarbons may reflect the structure o f hydrocarbonaceous adspecies during reaction with supported catalysts whil e it may transform into carbonaceous deposits on Pt-black. H-2 can split th em up to C-1 units, appearing as methane in the gas phase. Surface electron spectroscopy is in agreement with this latter assumption. (C) 2000 Elsevie r Science B.V. All rights reserved.