(377a) Light Olefin Oligomerization Via Isolated Ni2+ Sites on Well-Defined Lacunary Defects of Wells Dawson Polyoxometalate

Light olefin oligomerization is an important step to produce liquid fuels and high-value chemicals such as linear alpha olefins (LAOs). To avoid the inherent drawbacks of industrial processes that utilize homogeneous nickel complexes, there has been many attempts to isolate Ni active sites on well-defined solid hosts.^1,2 For example, metal organic frameworks (MOFs) have been used as a platform to bear single Ni(II) containing metal organic complexes within the well-defined framework. However, often the Ni (II) precatalyst requires activation by methyl-aluminoxane co-catalysts and liquid phase environments at nearly ambient temperatures to yield the best catalytic performance.³ Another approach to isolate Ni active sites is to exchange Ni²⁺ cations on paired Bronsted acid sites of zeolitic aluminosilicates. Despite the advantages of robust well-defined porous structures of zeolites, exchanged Ni²⁺ cations can suffer from catalytic deactivation due to the formation of bridging metal-olefin complexes between two proximal nickel cations.^4–7

To address these issues, we utilized lacunary defects of Wells Dawson (WD) polyoxometalates as molecular platforms to isolate a single Ni²⁺ cation as the Lewis acidic active site for light olefin oligomerization.⁸ The known synthetic technique which involves pH adjustments was used to create a vacancy on capping tri-tungsten oxide regions.⁹ Subsequently, addition of Ni²⁺ precursor occupied the formed lacunary defects of the WD polyoxometalate. Successful synthesis of the nickel substituted WD structure was confirmed by using various materials characterization techniques such as single crystal x-ray crystallography, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (NMR). Upon the dispersion of nickel substituted polyoxometalate species on the surface of mesoporous SBA-15, the catalytic contribution of substituted nickel cations was evidenced by the comparison between nickel free WD derivatives and blank catalyst support. After a cycle of light olefin oligomerization, the residual heavy oligomeric hydrocarbon species were able to be removed via a thermal treatment to regenerate the catalyst. The spent used Ni-WD catalyst which was exposed to ethylene or propylene was evaluated with solid-state NMR and TEM to showcase the remarkable stability of the catalyst. Furthermore, the kinetic parameters such as activation energy barrier, product distribution and olefin pressure dependence were measured to analyze the nature of substituted Ni sites on WD polyoxometalates through the comparison to literature reported solid Ni olefin oligomerization catalysts. In this presentation, we will highlight the application of nickel substituted WD polyoxometalate as a well-defined and regenerable light olefin oligomerization catalyst along with the kinetic analysis on the isolated Ni active sites.

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