Abstract
Ni functionalized metal organic frameworks (MOF) are promising heterogeneous ethene dimerization catalysts. Activities comparable to or higher than Ni-aluminosilicates have been reported in literature. However, unlike the Ni-aluminosilicates, those Ni-MOFs require a large excess of co-catalyst to initiate the dimerization process and some catalysts generate polymers which lead to catalyst deactivation. Herein, we report a series of Ni(II) and 2,2′-bipyridine-5,5′-dicarboxylate (bpy) functionalized UiO-67 MOF that catalyze the ethene dimerization reaction co-catalyst free. The catalysts were active for ethene dimerization (up to 850 mg butene gcat−1 h−1) after activation at 300 °C in 10 % O2 for 360 min and subsequent exposure to flowing ethene (P(ethene) =26 bar, 250 °C) for 240 min. The catalysts yielded up to 6 % conversion with 99 % selectivity to linear 1- and 2-butenes, which formed in non-equilibrated ratios. Overall, the test data indicate that all three linear butenes are formed on a single active site, in accordance with the Cossee-Arlman mechanism. Ex situ XAS and CO FT-IR spectroscopy studies point towards Ni monomers or, plausibly, low-nuclearity Ni-multimers, docked at bpy linkers with Ni-Ni distances > 4 Å, as the main active site for the ethene dimerization reaction.
Highlights
The industrial transition from naphtha to ethane as feedstock for ethene production has led to a shorter supply of propene and butene conventionally produced from oil [1]
(0.9) Ni-UiO-67- bpy11%, with Ni:bpy = 1:2, provides a large excess of bpy linkers with respect to Ni ions, resulting in almost exclusive presence of bpy-coordinated Ni2+ moieties inside the metal-organic framework (MOF) scaffold: comparing the (0.9) Ni-UiO-67- bpy11% XAS data with the ones obtained for a series of well-characterized bpy-OAc Ni2+ com plexes, we found the best agreement in both the XANES and EXAFS region with a Ni(OAc)2(H2O)2(bpy) model complex, structurally char acterized by single crystal XRD
The results obtained in this study confirm the potential of co-catalyst free Ni-UiO-67-bpy as ethene dimerization catalysts
Summary
The industrial transition from naphtha to ethane as feedstock for ethene production has led to a shorter supply of propene and butene conventionally produced from oil [1]. The synthetic diversity of these materials enables control over density of active sites and pore size, and the ability to incorporate so phisticated chemical functionalities as part of the framework [6,7] This strategy includes installing homogeneous catalysts onto the organic li gands [8,9], on the metal clusters [10,11,12] or encapsulating metal par ticles [13]. The bpy linker was reported to be exchanged with the biphenyl linkers of UiO-67 and functionalized with Ni2+ This catalyst was active for ethene oligomerization and products ranging from C4 – C18 was observed. Brogaard et al performed Density Functional Theory (DFT) calculations of ethene oligomerization in another microporous zeolite-based catalyst, Ni-SSZ-24, and found that the Cossee-Arlman mechanism is energetically favored compared to the metallocycle mechanism in this catalyst. Detailed characterization of fresh and used catalyst (SEM-EDS, N2 adsorption, XRD, TGA, FTIR, XAS) was performed to identify the active site and to verify the structural stability of the catalyst under reaction conditions
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