Cooperative Effect of Pd²⁺ and Pd⁰ in Selective Hydrogenation of Nitrile Rubber

The selective hydrogenation of nitrile butadiene rubber (NBR) to hydrogenated nitrile butadiene rubber (HNBR) plays a crucial role in automotive, aerospace, and other industries. However, this reaction faces challenges, including low activity, poor stability, and an unclear deactivation mechanism of the catalyst. In this work, Pd/Al2O3 with a Pd2+ content exceeding 60% was synthesized through a one-step process involving high-temperature pyrolysis and rapid cooling. Under extremely mild reaction conditions of 30 °C and 0.1 MPa, the Pd/Al2O3 catalyst achieves efficient hydrogenation of NBR, with a hydrogenation degree of NBR exceeding 99 and 100% selectivity for HNBR within 2 h. Through a series of characterizations and experimental analyses, it was revealed that the primary reason for catalyst deactivation is the change in the electronic structure of Pd, specifically, the in situ reduction of some Pd2+ in the catalyst to Pd0 during the reaction process. By precise control of the proportion of Pd2+ in the catalyst, a linear relationship between the Pd2+ content and the hydrogenation degree of NBR was established. The synergistic effect of Pd2+ and Pd0 synergistically reduces the reaction energy barriers, thereby enhancing the catalyst activity.