Piperidine hydrogenolysis on a commercial hydrocracking catalyst: III. The effects of zeolite unit cell size, catalyst sulfur content, and coke deposition on catalyst activity and deactivation

Abstract

A group of fresh, deactivated, and deactivated-regenerated commercial hydrocracking catalysts was characterized using piperidine hydrogenolysis as a probe reaction at 301C, hydrogen partial pressures of about l6 atm, and initial concentrations of piperidine ranging from 4.14 × 10 −3 11.86 × 10 −3 g mol/liter. The first part of the study examined the effects of the zeolite unit cell size on the piperidine hydrogenolysis activity and catalyst deactivation and revealed that over the range of unit cell size examined (24.35-24.56 Å), adsorption equilibrium constants associated with the acidic and metallic catalyst functions decreased while reaction rate constants increased with increasing unit cell size. The increase of acidic function activity with unit cell size is explained in terms of the total number of potential acid sites present per unit cell. The increase of metallic function activity with unit cell size is suggested to reflect mostly changes of the metallic function upon regeneration. Different types of sites are proposed to exist on the metallic catalyst function in order to account for the results. An inverse relationship between catalyst deactivation and unit cell size was generally observed for both catalyst functions. These results point to the zeolite unit cell size as a possible parameter for correlating catalytic activity as well as selectivity, especially for the acidic catalyst function. The second part of the study examined the effects of wt% sulfur and wt% carbon at low levels of sulfur and coke on the piperidine hydrogenolysis activity and showed that the regenerated catalysts with constant zeolite unit cell size (≈24.40 Å and low levels of coke (<0.8 wt%) had similar acidic function activities. The acivity correlation of the metallic function on these regenerated catalysts with low levels of sulfur (<0.4 wt%) also indicated the presence of two different types of metal sites. One showed a decrease in activity with increasing sulfur content and a possible dependency on the regeneration procedure while the activity of the other site was invariant. The activity of the metallic and acidic functions of the catalyst was also examined at high levels of sulfur and coke and constant unit cell size. For this study a series of deactivated samples was used. The metallic function activity of these catalysts was found to decrease with the commercial time on stream. Between 2.2 and 2.6 wt% sulfur, the activity of the metallic function decreased linearly with the sulfur content on the catalyst. At higher levels of sulfur (>2.5 wt%) the deactivated catalysts had similar metallic function activities. Between 2 and 6 wt% carbon, the activity of the acidic function decreased linearly with the carbon content on the catalyst. At low levels of coke (< 1.5 wt%), the effect of the unit cell size on acidic activity prevailed. The wt% carbon on the deactivated as well as the regenerated catalysts was found to have no effect on the catalyst deactivation rates observed during the reactions.