Catalytic activation of ethylene C-H bonds on uniform d 8 Ir ( I ) and Ni ( II ) cations in zeolites : toward molecular level understanding of ethylene polymerization on heterogeneous catalysts

Abstract

The homolytic activation of the strong C-H bonds in ethylene is demonstrated, for the first time, on d8 Ir(I) and Ni(II) single atoms in the cationic positions of zeolites H-FAU and HBEA under ambient conditions. The oxidative addition of C2H4 to the metal center occurs with the formation of a d6 metal vinyl hydride, explaining the initiation of the Cossee-Arlman cycle on d8 M(I/II) sites in the absence of pre-existing M-H bonds. Under mild reaction conditions (80-220oC, 1 bar), the catalytic dimerization to butenes and dehydrogenative coupling of ethylene to butadiene occurs over these catalysts. Butene-1 is not converted to butadiene under the reaction conditions applied. Post-reaction characterization of the two materials reveals that the active metal cations remain site-isolated whereas deactivation occurs due to the formation of carbonaceous deposits on the zeolites. Our findings have significant implications for the molecular level understanding of ethylene conversion and the development of new ways to functionalize C-H bonds under mild conditions. Zeolite-supported transition metals (single atoms, clusters, nanoparticles, etc.) represent an important class of materials with uses in the chemical industry, emissions controls, and as model systems to derive structure-function properties in catalysis.1-9 Among them, d8 metals such as Ni(II), Rh(I), Ir(I), Pt(II), and Pd(II) have been the focus of many studies to better understand the genesis, speciation, and stability of such species for reactions such as hydrogenations, oxidations, as well as ethylene transformation (diand oligomerization to butenes and higher oligomers).10-13 For example, it was shown first in the 1950s that Rh(I)(CO)2 and Ir(I)(CO)2 species can be stabilized on oxide supports14-15 and are active for ethylene conversion to butenes at room temperature, retaining their site-isolated nature after catalysis.16-18 The Rh ligand environment is tunable and hydrogen promotes butene formation despite not directly participating in the dimerization reaction (i.e., 2C2H4 \uf0e0 C4H8).12, 17-18 This effect was explained in some studies by H2 enhancing butene desorption on (Rh(C2H4)2/HY).16 Recently, however, the hydrogen partial pressure dependence of ethylene dimerization was systematically measured on Rh(CO)2, Rh(CO)(C2H4), Rh(CO)(H),17 and Rh(NO)212 complexes supported on HY zeolites. Positive reaction orders of ~0.7-1 confirmed that hydrogen indeed promotes dimerization, where H2 was shown to improve the rate of ethylene dimerization up to ~10 fold.12,17 This was attributed to the formation of metal-hydride-supported species (observed and characterized experimentally12,17,18) which provide a low-energy pathway for dimerization via facile insertion of pi-coordinated ethylene into the M-H bond to form an M-Ethyl moiety which subsequently migrates into another pi-coordinated ethylene to form a Rh-Butyl species prior to facile β-H abstraction to produce butene-1.12 This attribution was subsequently supported for ethylene dimerization on Ni/BEA, although Ni-H species were not observed directly.19 Until now, it remained unclear how ethylene, in the absence of M-hydride species, can polymerize considering the importance of M-H intermediates in the Cossee-Arlman mechanism. Theoretical studies have identified potential mechanisms for ethylene dimerization on Ni/BEA where the metallocycle, protontransfer, and Cossee-Arlman mechanisms were compared.20 Also considered was the non-catalytic formation of a nickel vinyl intermediate via the heterolytic activation of a C-H bond over Ni(II)-O bond followed by the formation of an active Ni center.20 In this study, we demonstrate the : 1). Preparation and characterization of highly uniform d8 metal species. Ni(II) was selected because it has been a challenge to prepare well-defined uniform Nizeolite species. We have previously prepared d8 Pt(II) and Pd(II) species9 in zeolite uniformly and thus transferred this approach to a Ni/BEA system in order to unravel detailed structure catalyticproperty relationships for the historically important system for ethylene polymerization. We also employ the well-defined square planar d8 Ir(I)(CO)2 complex anchored in zeolite FAU (like Ni(II)/FAU) because it grafts uniformly in zeolite and also has CO groups which, due to their high molar extinction coefficients and well-resolved nature, allow us to observe ligand changes with enhanced resolution. 2). We obtain the reactivity for ethylene couplings on those materials, showing similar trends for both d8 metals 3). We resolve a longstanding uncertainty in heterogeneous ethylene polymerization, one of the largest catalytic processes. Though supported metal ions (d8 like Ni(II), Ir(I), Pd(II) or d4 Cr(II) perform this reaction without the initiator/co-catalyst, the mechanism for ethylene polymerization initiation and the relevant intermediates involved have remained elusive for the last 50 years. We resolve these uncertainties using state-of-the-art infrared studies supported by microscopy and solid-state NMR measurements for d8 metal cations on solid supports. In short, ethylene polymerization starts with the homolytic activation of the C-H bonds of ethylene on extremely electrophilic d8 M sites, resulting in the formation of d6 metal vinyl hydride complexes which further react with ethylene to form a vinyl ethyl d6 metal fragment. From this fragment, butene-1 can form either via direct reductive elimination or a Cossee-Arlman type step involving alkyl chain growth through alkyl migration and insertion into M-ethylene bonds. Though reported for other d8 metals, it is not straightforward to generate uniform Ni(II) species since they may graft to both silanol nests and various extra-framework zeolite positions, evidenced by IR spectroscopy of CO adsorption.19 This brought into question the true active center for ethylene oligomerization activ-