Ethylene oligomerization under confinement using supported Cr(ii) and Cr(iii) catalysts

Abstract

Supported Cr(II) and Cr(III) catalysts have emerged as versatile systems for the selective oligomerization of ethylene, offering advantages such as easy separation, catalyst recyclability, and reduced polymer formation. However, their performance can be significantly influenced by factors such as pore confinement, active site accessibility, and metal–support interactions, which leads to an altered selectivity as compared to their homogeneous counterparts. Herein, to better understand and potentially overcome these limitations, we investigated how support morphology and confinement influence the behavior of immobilized Cr catalysts during ethylene oligomerization. Supported catalysts were prepared by selective immobilization of the Cr(III) complexes [1-methyl-3-(2-amido-N-(2,6-diisopropylpheny-1-yl)phen-1-ylimidazol-2-ylidene)₂Cr₂-(μ-Cl)₄] (Cr1), [1-methyl-3-(2-amido-N-(2,6-diisopropylpheny-1-yl)phen-1-ylimidazol-2-ylidene)Cr(benzyl)₂(THF)] (Cr2), and the Cr(II) complex [(CAAC)₂CrCl₂] (Cr3, CAAC = 1-(2,6-diisopropylpheny-1-yl)-3,3,5,5-tetramethyltetrahydropyrrol-2-ylidene), which are all non-selective for ethylene oligomerization in solution, inside the mesopores of ordered mesoporous silica (OMS) materials with varying pore sizes (OMS_66Å, OMS_57Å, OMS_28Å), as well as amorphous silica (Si60). The activity and selectivity of the supported catalysts were assessed in comparison to the parent homogenous catalysts. In the presence of methylalumoxane (MAO) as cocatalyst, the immobilized catalysts exhibited lower catalytic activity (0.1–3.1 kg mol Cr⁻¹ h⁻¹ bar⁻¹) compared to the homogeneous catalysts (6.9–8.3 kg mol Cr⁻¹ h⁻¹ bar⁻¹), yet with significant reduction in polyethylene (PE) byproduct formation. Upon activation with MAO, particularly the OMS-immobilized catalyst Cr1@OMS_28Å as well as Cr2@OMS_28Å allowed for the selective synthesis of linear α-olefins (LAOs) ≤ C₁₂.