Selective 1,3-butadiene dimerization to 1,3,7-octatriene catalyzed by a heterogeneous Pd catalyst supported on a sterically encumbered NHC polymer

Abstract

As a significant chemical feedstock, 1,3,7-octatriene (OCT) has so far been synthesized mainly via homogeneous Pd catalyst systems, with no successful application of heterogeneous catalysts reported to date. To overcome the challenges associated with catalyst separation/recovery and the high cost of noble-metal, in this study, we developed a novel heterogeneous catalytic system. Using Scholl coupling method, sterically hindered N-heterocyclic carbene (NHC) ligand was incorporated into a hypercrosslinked polymer network, which served simultaneously as both the catalyst support and the solid ligand in the Pd-catalyzed dimerization of 1,3-butadiene with isopropanol. The heterogenous Pd-supported hypercrosslinked polymer catalyst (denoted as Pd@HCP-NHC-B) was formed in situ during the reaction and demonstrated outstanding catalytic activity. Under optimized conditions, complete conversion of 1,3-butadiene was achieved with a selectivity of up to 93.5% toward the target product OCT, which is comparable to that its homogeneous counterpart. Importantly, the catalyst could be easily separated from the reaction mixture and maintained consistent activity and selectivity over at least 10 consecutive cycles. Characterization studies confirmed that the morphology of the catalyst and the local coordination environment of the metal centers remained unchanged after agglomeration, demonstrating the high stability of the catalyst. This breakthrough paves the way for scalable and industrially feasible heterogeneous catalysts in 1,3-butadiene dimerization.