Authors

Abstract

The inherent crystallinity of metal-organic framework (MOF) catalysts offers the possibility to understand the structure of the active site at the molecular level. This property is often lacking in traditional amorphous supports. Cr-SIM-NU-1000, a MOF-supported Cr3+ heterogeneous catalyst, is shown to be competent for ethylene polymerization after activation with AlEt2Cl (DEAC), producing crystalline linear polyethylene (PE). The polymer produced has a low polydispersity (D = 2.0), in marked contrast to the Phillips supported chromium catalyst, Cr@SiO2 (D similar to 8-65). Cr-SIM-NU-1000 achieves a turnover frequency of 2.6 x 10(3) h(-1) under 40 bar ethylene pressure at room temperature, with corresponding PE productivity of 1.3 x 10(5) g PE mol(-1) Cr per h. Single crystal X-ray diffraction (SC-XRD) of the pre-catalyst was conducted by collecting a structure of alkyl aluminum (DEAC) co-catalyst treated Cr-SIM-NU-1000. This crystal structure provides insight into the interactions between DEAC co-catalyst and Cr active site, revealing a Cr-C bond after treatment with DEAC. Furthermore, DEAC is crystallographically resolved at the terminal oxy-ligands of the node and likely also exists within the window pores of the framework between nodes, based on electron density mapping. Cr-SIM-NU-1000 offers the opportunity to study a structurally well-defined olefin polymerization system, with atomically precise characterization of the pre-catalyst structure. This allows the proposal of a mechanism and feeds into future development of next-generation heterogeneous catalyst systems.