Conversion of methylcyclopentane and acyclic hexanes over supported platinum catalysts: II. Partial pressure and temperature effects on the formation of methylcyclopentane from the three acyclic hexanes

Abstract

The sequential reaction model proposed in Part I ( J. Cutal. 112, 290 (1988)) of this series for the isomerization of acyclic hexanes via five-ring intermediates is supported by studies of the partial pressure and reaction temperature effects on methylcyclopentane (MCP) formation from n-hexane ( nC 6), 2-methylpentane (2MP), and 3-methylpentane (3MP). MCP concentration is found to increase linearly with increasing p hexane/P H2 ratio as predicted by the model. Enhanced MCP formation with increasing hexane partial pressure and with decreasing hydrogen partial pressure results from an increase in the surface concentration of the five-ring intermediates. Enhanced surface coverage by five-ring intermediates is accompanied by a decrease in hydrogenolysis presumably by reducing the surface coverage of the 3C intermediates. The sequential reaction model for the acyclic hexanes also predicts a linear relationship between ln[MCP] and 1 T with the slope of the line giving the activation energy difference between ring closure and ring opening reactions. Experimental results are in excellent agreement with this prediction. Activation energy differences between ring closure and ring opening reactions indicate that the five-ring closure between two primary carbon atoms (2MP and 3MP) has a somewhat higher activation energy than five-ring closure between a primary and a secondary carbon atom ( nC 6). Differences between five-ring closures of 2MP and 3MP and nC 6 are also evidenced by changes in reaction rates with decreasing hydrogen partial pressure and increasing reaction temperature. The relative nC 6 concentration from either MCP ring opening or 2MP and 3MP isomerization decreases with time on stream and with decreasing hydrogen partial pressure. These observations, together with results of MCP ring opening over Pt/TiO 2 catalysts, suggest that a carbonaceous layer deposited on the Pt surface during reaction sterically hinders the formation of MCP-like five-ring intermediates having tertiary-secondary CC bonds bound to the Pt surface.