Industrial & Engineering Chemistry Research, Vol.44, No.6, 1677-1687, 2005
Rejuvenation of moisture-exposed Pd/delta-Al2O3 catalysts for isoprene-selective hydrogenation: Correlation between Pd dispersion and catalytic properties
Exposure of commercial delta-alumina-supported Pd catalysts to moisture is unavoidable. Besides handling the catalysts with care, development of rejuvenation procedure for moisture-exposed catalyst is also necessary to minimize the economical loss. By use of in situ EXAFS (extended X-ray absorption fine structure) spectroscopy and CO chemisorption, we observed (1) moisture adsorbed on the catalyst facilitates the mobility of palladium oxides, leading to formation of bigger Pd clusters, and (2) moisture adsorbed on PdO could not be removed at temperatures of 200 degrees C or below, whereas it can be removed significantly in flowing air at 550 degrees C without growth in PdO clusters. In contrast, in the flow on nitrogen at 550 degrees C, the PdO is decomposed to Pd metal, which aggregates to large clusters. The impact of water exposure and the efficiency of rejuvenation were further examined by isoprene hydrogenation at a weight hourly space velocity (WHSV) of 36 h(-1) (g of feed/(h g of catalyst)), 40 degrees C, and 30 atm total pressures. The results indicate that moisture saturation caused the decrease in isoprene conversion and selectivity to 2-methyl-2-butene (2M2B) by at least 20%. The losses of selectivity and activity can be recovered to about 60% by flowing air at 550 degrees C to remove the adsorbed moisture. Moreover, the test results show that isoprene conversion increases with Pd dispersion both at start of run and at nearly steady state, whereas the selectivity to 2-methyl-2-butene decreases with Pd dispersion at start of run and reaches a steady state as the time on stream is increased. The change of selectivity to 2-methyl-2-butene can be explained on the basis of FT-IR characterizing CO adsorbed on the Pd catalysts; Pd sites for CO chemisorption can be distinguished as bridging and terminal sites, and the terminal site has a higher tendency to convert isopentene to isopentane and is more vulnerable to be poisoned by coke deposition.