Ethylene polymerization with a crystallographically well-defined metal–organic framework supported catalyst

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 (Đ = 2.0), in marked contrast to the Phillips supported chromium catalyst, Cr@SiO₂ (Đ ∼ 8–65). Cr-SIM-NU-1000 achieves a turnover frequency of 2.6 × 10³ h⁻¹ under 40 bar ethylene pressure at room temperature, with corresponding PE productivity of 1.3 × 10⁵ 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.