The coking of a highly porous commercial Pt/γ-Al2O3 catalyst via an olefinic precursor, viz., 1-hexene, was investigated in a CSTR over a range of temperatures and pressures that encompass low density subcritical as well as dense supercritical regions of operation. Temporal changes in catalyst effectiveness factor, a measure of catalyst activity, are presented for coking in subcritical and supercritical reaction mixtures at three different reactor temperatures between 1.01 and 1.10 Tc The coke laydown during each run along with coked catalyst characteristics such as BET surface area, pore volume, and pore volume distribution are also reported. The maintenance or decay of catalyst activity is dependent upon the relative rates of coke formation and of coke extraction through the catalyst pores. Each of these rates is dictated by density and temperature. At each temperature studied, there is insignificant coke extraction in subcritical reaction mixtures and hence catalyst effectiveness factor decreases with time. Also, when 1-hexene isomerization is kinetically controlled, there is volume loss in both high activity (20-80 Å) and low activity (100-600 Å) pores indicating coke laydown throughout the catalyst pore structure. In low to moderate supercritical density reaction mixtures, the effectiveness factor decreases rapidly accompanied by reduced coke laydown and virtually no volume loss in the high activity pores. This is attributed to extensive pore mouth plugging of the high activity pores caused by increased coke formation rates relative to coke extraction rates. In dense supercritical reaction mixtures (>1.7 ρ{variant}c, the enhanced solubilities of the coke compounds in the reaction mixture alleviate pore mouth plugging of the high activity pores, as evidenced by an increase in the effectiveness factor and by extensive volume and surface area losses in the high activity pores. At the highest density studied (2.53 ρ{variant}c, while effectiveness factors are smaller than subcritical values at 1.01 Tc, up to two-fold enhancements in temporal effectiveness factors are seen at 1.10 Tc. A theoretical model is needed to better understand the density and temperature effects on the physicochemical processes underlying coke deposition with simultaneous extraction in porous catalysts. © 1991.
