Pd/δ-Al2O3 Catalysts for Isoprene Selective Hydrogenation: Regeneration of Water-Poisoned Catalysts

Water-containing isoprene was partial hydrogenated over δ-alumina-supported eggshell Pd catalysts with a weight-hourly space velocity (WHSV) of 18 h - 1 (grams of feed per hour × grams of catalyst) at 40 °C and 30 atm (30.39 kg/cm 2 ) total pressure. The isoprene conversion decreased with time on stream and became almost time invariant at about 50 h, with a value of 20%. The activity of the water-poisoned catalysts was only slightly recovered after the water-free isoprene was introduced; by this, the conversion was increased only to 24%. In contrast, by hydrogen treatment prior to the introduction of water-free feed, the water-poisoned catalyst was greatly reactivated. The improvement in activity was also influenced by temperature. The number of Pd sites available for CO chemisorption was increased and, thus, the catalyst activity was improved by a raise in temperature to 200 °C, then declined. At nearly steady state, the isoprene conversion by catalysts reactivated at 200 °C is about 93% of that by catalysts without water-poisoning. The loss of the catalyst activity was thought to be caused by the Pd agglomeration during hydrogen reactivation. EXAFS (extended X-ray absorption fine structure) spectroscopy results indicated that the average Pd−Pd coordination number increased from 6.4 to 8.3 after reactivation of the catalyst with hydrogen at 400 °C.