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.