Hydrocarbon reactions on supported iridium catalysts

Abstract

The reactions of ethane, n-butane, isobutane, neopentane, neohexane, 2,3-dimethylbutane, and methylcyclopentane have been studied over a range of dispersed iridium catalysts in which the iridium was supported on γ-alumina or Aerosil silica, and d̄Ir lay in the range ≤ 1 to 20 nm. The catalysts were characterized by electron microscopy, hydrogen adsorption, and temperature-programmed desorption of hydrogen. Hydrocarbon hydrogenolysis was the sole reaction pathway, except in the reaction of neopentane over catalysts with d̄Ir of 7 and 20 nm, where some isomerization to isopentane was observed, and in the case of isobutane, where there was some inferential evidence for isomerization to n-butane prior to hydrogenolysis. The hydrocarbons fell into one of two classes depending upon the activation energy and the type of hydrogenolysis reaction occurring. Reaction in a C2-unit mode (ethane the archetypal hydrocarbon) occurred with an activation energy in the region of 175 kJ mol−1, and was the reaction mode for CI-CI, CI-CII, and CII-CII bond types. Reaction in an iso-unit mode (neopentane the archetypal hydrocarbon) occurred with an activation energy in the region of 235 kJ mol−1, and was the reaction mode for CI-CIV, CI-CIII, and CII-CIII bond types. (CI indicates a primary carbon, etc.) The activation energy and the frequency factor (expressed as rate per surface iridium atom) for reaction in the C2-unit or the iso-unit mode was independent of d̄Ir. Provided the hydrogen pressure was kept sufficiently high, all reaction rates were proportional to P1HC P−3H2. The reaction mechanism is discussed and is compared with the behavior of the corresponding reactions over platinum catalysts.