Amino-tunable metal–organic frameworks as platforms for Cr–PNP-catalyzed selective ethylene oligomerization

Grafting molecular Cr–PNP complexes onto the amino groups of metal–organic frameworks (MOFs) constitutes a promising strategy for anchoring selective ethylene oligomerization catalysts on a crystalline, porous support; however, a conflict exists between high amino density and framework porosity. Herein, we employ a mixed-ligand approach in which 2-amino-1,4-benzenedicarboxylic acid and terephthalic acid were combined at four molar ratios to construct two large-pore frameworks with tunable amino content, UiO-66 and MIL-101(Cr) (denoted MixNU and MixNM). The amino groups were phosphinated with chlorodiphenylphosphine and subsequently metalated with Cr(III) to afford the grafted catalysts MixNUPCr and MixNMPCr. Multiple characterization techniques confirmed that the framework structure and crystallite morphology were preserved throughout phosphination and metalation. Pre-formed Cr–PNP sites exhibited markedly higher catalytic activity than chromium generated in situ or introduced by impregnation, and produced no polyethylene. Activity varied with amino content and peaked at an amino/terephthalate molar ratio of 2:3: MixNUPCr-2 reached 2.49 × 10 5 g·mol −1 ·h −1 with a C6 + C8 selectivity of 71.14%, while MixNMPCr-2 reached 1.01 × 10 5 g·mol −1 ·h −1 with a C6 + C8 selectivity of 62.07%. MixNUPCr-2 retained 82% of its peak activity at 60 °C and exhibited no appreciable loss of activity or crystallinity over four cycles. Density functional theory calculations reproduced the metallacycle mechanism and confirmed that ring expansion of the chromacyclopentane intermediate is favored over 1-butene release, consistent with the C6/C8-rich product distribution observed experimentally.