Nano-ceria-zirconia promoter effects on enhanced coke combustion and oxidation of CO formed in regeneration of silica-alumina coked during cracking of triisopropylbenzene

Effects of Ce0.68Zr0.32O2 physically mixed with or impregnated on amorphous silica-alumina (SA) on enhanced combustion of coke and oxidation of CO formed in the regeneration of the catalysts coked during catalytic cracking of 1,3,5-triisopropylbenzene (TiPB) were studied. SA with about 535 m(2)/g BET surface area was synthesized using a co-precipitation method and ammonium exchanged. 6.0 wt% Ce0.68Zr0.32O2 on the silica-alumina was prepared by impregnation of the sample with an aqueous solution of cerium and zirconium nitrates and designated as Ce0.68Zr0.32O2/SA. Nanoparticles of Ce0.68Zr0.32O2, with around 12 nm crystallite size, were also prepared by a microwave induced combustion synthesis method and physically mixed with SA as another catalyst sample, i.e. Ce0.68Zr0.32O2 center dot SA. The catalysts were characterized by SEM, EDS, XRD, BET, AAS, H-2-TPR and ammonia TPD techniques. After in situ calcination at 475 degrees C for 3 h, the samples were coked by exposure to a gaseous stream of 5.0 vol% TiPB in Ar at 350 degrees C for 40 min. Temperature-programmed oxidation (TPO) method was employed to study the onset temperature of the coke combustion and CO oxidation during the regeneration of the coked catalyst in 2.0 vol% O-2/N-2 feed. At 3 min of cracking, TiPB cracking activities of both Ce0.68Zr0.32O2/SA and Ce0.68Zr0.32O2 center dot SA samples are about the same and 5% lower than that of SA. The selectivity of cumene, as a deep cracking product, decreases in the order of SA > Ce0.68Zr0.32O2 center dot SA > Ce0.68Zr0.32O2/SA concomitant with the density of acid sites of the catalysts. As compared to the SA, a decline of about 15.2% in coke formation is observed for the Ce0.68Zr0.32O2/SA sample. In addition, the Ce0.68Zr0.32O2 promotion of SA by physical mixing and impregnation lowers the CO evolution in the catalysts regeneration by about 51 and 61%, respectively. The highly active Ce0.68Zr0.32O2 oxygen is responsible for the higher conversion of CO to CO2 On the catalysts containing the promoter. Furthermore, the Ce0.68Zr0.32O2/SA sample, impregnated with Ce0.68Zr0.32O2 promoter, lowers the onset temperature of coke combustion by about 130 degrees C. This may be attributed to tight contact of the coke with active surface oxygen of the promoter. (C) 2008 Elsevier B.V. All rights reserved.