Selective linear ethylene oligomerization over nickel-containing zeotypes with tetravalent framework heteroatoms†

Heterogeneously catalyzed ethylene oligomerization to higher olefins remains an industrial challenge due to the difficulties to effectively activate ethylene and simultaneously ensure high product selectivities. Nickel aluminosilicate zeolites have shown promise as alternatives to homogeneous catalysts, but the strong Brønsted acidity of these materials typically results in a broad product distribution and the accumulation of heavy oligomers within the channels and cages of porous zeolites. Herein, we report the positive impact on selectivity to linear olefinic products with the replacement of Brønsted acidic zeolites by a family of Lewis acidic Beta zeotypes as supports for nickel. Our findings reveal that the acidic nature of the zeotypes is fundamental to direct the successful incorporation of nickel into active species. Sn-, Hf- and Zr-Beta stand out as compared to Ti- and Ge-Beta in terms of acid strength, nickel ion-exchange capacity, and ultimately, catalytic activity. All active materials yield remarkable selectivities (>90%) to linear olefins. In contrast, conventional Ni/Al-Beta, having Brønsted acidity, provides a complex product spectrum due to the promotion of competitive reaction pathways (e.g., skeletal isomerization, cracking and hydrogen transfer reactions). Furthermore, the retained hydrocarbon species need higher oxidation temperatures to be removed over Ni/Al-Beta as compared to Ni-containing Lewis acidic zeotypes, hinting towards a milder coking deactivation of the latter materials. These insights show the potential for improving ethylene oligomerization selectivity and catalyst stability with the utilization of a new class of zeotypes without strong Brønsted acidity, but with the ability to anchor transition metals.