Promotional effect of chloride on iridium-iron catalyst for chemo-selective hydrogenation of unsaturated ketones to unsaturated alcohols

The Cl − -containing Ir–FeO x(−Cl) /ZrO 2 catalyst (Ir: 4 wt%, Fe/Ir = 0.5, FeCl 3 precursor) was proved to be an effective one towards benzalacetone hydrogenation to 4-phenyl-3-buten-2-ol. The Ir–FeO x(−Cl) /ZrO 2 catalyst showed a high selectivity to 4-phenyl-3-buten-2-ol (72% at a 73% conversion level) and the highest activity (TOF m = 18.4 min −1 (based on total Ir), TOF s = 90 min −1 (based on surface Ir)) among all the reported heterogeneous catalysts. This catalyst was also applicable to selective hydrogenation of various other unsaturated ketones and unsaturated aldehydes. Lower substrate concentration gave higher yields, and the yields of 4-phenyl-3-buten-2-ol and 2-cyclohexen-1-ol reached 70% and 48%, respectively, under diluted conditions. The presence of Cl was essential in the selectivity, and external addition of Cl to Ir–FeO x(−NO3) /ZrO 2 (Fe(NO 3 ) 3 precursor) by the treatment with NH 2 OH·HCl gave comparably effective performance. While the Ir–FeO x(−Cl) /ZrO 2 catalyst gradually lost its activity in reuses due to the structural evolution and the loss of active sites, the used catalyst can be regenerated by the combination of calcination and treatment with NH 2 OH·HCl. Based on the difference of catalytic performance among Ir/ZrO 2 , Ir–FeO x(−NO3) /ZrO 2 , Ir–FeO x(−Cl) /ZrO 2 and deactivated Ir–FeO x(−Cl) /ZrO 2 , the structure-performance relationship was clarified. From the characterizations with TEM, XPS, XAFS and FT-IR, combined with the reaction kinetics and activity of each catalyst, the active sites were proposed as Ir-rich Ir–Fe alloy species modified with [Fe m+ -Cl] which is a Lewis acid as an effective adsorption site for C=O group.