OLEFINIC OLIGOMER AND COSOLVENT EFFECTS ON THE COKING AND ACTIVITY OFA REFORMING CATALYST IN SUPERCRITICAL REACTION MIXTURES

During investigations of 1-hexene isomerization on a Pt/gamma-Al2O3 ca talyst, hexene oligomers are found to form in the fluid phase. The tot al amount of oligomers increased by roughly an order of magnitude with isothermal increases in pressure from subcritical to supercritical va lues. These oligomers are significant coke precursors, increasing the coke formation and catalyst deactivation rates. Dilution of the hexene feed with an inert cosolvent like n-pentane, such that the reaction m ixture is at supercritical conditions, reduces oligomer concentrations while maintaining the in situ extraction of the coke-forming compound s. Consequently, coke laydown is significantly reduced resulting in in creased isomerization rates and decreased deactivation rates. Although oligomer formation and coke laydown decrease continuously with cosolv ent addition, there exists an optimum cosolvent fraction beyond which the isomerization rate becomes limited by 1-hexene concentration and h ence decreases. It was determined that ppm levels of organic peroxides in the hexene feed, formed as a result of exposure to air during eith er storage or the experiments, aid the formation of hexene oligomers i n the fluid phase. Pretreatment of the hexene feed with activated alum ina results in the virtual elimination of the peroxides and a severalf old decrease in total oligomer concentration. Our results indicate tha t peroxide removal from the hexene feed and cosolvent addition can sig nificantly improve isomerization rates, catalyst activity maintenance, and pore accessibility in supercritical reaction mixtures. (C) 1995 A cademic Press, Inc.