Mechanistic Investigations of Ethene Dimerization and Oligomerization Catalyzed by Nickel-containing Zeotypes
Dimerization and oligomerization reactions of alkenes are promising catalytic strategies to convert light alkenes, which can be derived from light alkane hydrocarbons (ethane, propane, butane) abundant in shale gas resources, into heavier hydrocarbons used as chemical intermediates and transportation fuels. Nickel cations supported on aluminosilicate zeotypes (zeolites and molecular sieves) selectivity catalyze ethene dimerization over oligomerization given their mechanistic preference for chain termination over chain propagation, relative to other transition metals commonly used for alkene oligomerization and polymerization reactions. Ni-derived sites initiate dimerization catalytic cycles in the absence of external activators or co-catalysts, which are required for most homogeneous Ni complexes and Ni2+ cations on metal organic frameworks (MOFs) that operate according to the coordination-insertion mechanism, but are not required for homogeneous Ni complexes that operate according to the metallacycle mechanism. Efforts to probe the mechanistic details of ethene dimerization on Ni-containing zeotypes are further complicated by the presence of residual H+ sites that form a mixture of 1-butene and 2-butene isomers in parallel acid-catalyzed pathways, as expected for the coordination-insertion mechanism but not for the metallacycle mechanism. As a result, the mechanistic origins of alkene dimerization on Ni cations have been ascribed to both the coordination-insertion and metallacycle-based cycles. Further, different Ni site structures such as exchanged Ni2+, grafted Ni2+ and NiOH+ cations are proposed as precursors to the dimerization active sites, based on analysis of kinetic data measured in different kinetic regimes and corrupted by site deactivation, leading to unclear and contradictory proposals of the effect of Ni precursor site structures on dimerization catalysis.
Dimerization of ethene (453 K) was studied on Ni cations exchanged within Beta zeotypes in the absence of externally supplied activators, by suppressing the catalytic contributions of residual H+ sites via selective pre-poisoning with Li+ cations and using a zincosilicate support that contains H+ sites of weaker acid strength than those on aluminosilicate supports. Isolated Ni2+ sites were predominantly present, consistent with a 1:2 Ni2+:Li+ ion-exchange stoichiometry, CO infrared spectroscopy, diffuse reflectance UV-Visible spectroscopy and ex-situ X-ray absorption spectroscopy. Isobutene serves a kinetic marker for alkene isomerization reactions at H+ sites, which allows distinguishing regimes in which 2-butene isomers formed at Ni sites alone, or from Ni sites and H+ sites in parallel. 1-butene and 2-butenes formed at Ni sites were not equilibrated and their distribution was invariant with ethene site-time, revealing the primary nature of butene double-bond isomerization at Ni sites as expected from the coordination-insertion mechanism. In-situ X-ray absorption spectroscopy showed that the Ni oxidation state was 2+ during dimerization, also consistent with the coordination-insertion mechanism. Moreover, butene site-time yields measured at dilute ethene pressures (<0.4 kPa) increased with time-on-stream (activation transient) during initial reaction times, and this activation transient was eliminated at higher ethene pressures (≥ 0.4 kPa) and while co-feeding H2. These observations are consistent with the in-situ formation of [Ni(II)-H]+ intermediates involved in the coordination-insertion mechanism, as verified by H/D isotopic scrambling and H2-D2 exchange experiments that quantified the number of [Ni(II)-H]+ intermediates formed.
The prevalence of the coordination-insertion cycles at Ni2+ cations provides a framework to interpret the kinetic consequences of the structure of Ni2+ sites that are precursors to the dimerization active sites. Beta zeotypes predominantly containing either exchanged Ni2+ cations or grafted Ni2+ cations show noteworthy differences for ethene dimerization catalysis. The deactivation transients for butene site-time yields on exchanged Ni2+ cations indicate two sites are involved in each deactivation event, while those for grafted Ni2+ cations indicate involvement of a single site. The site-time yields of butenes extrapolated to initial time, and then further extrapolated to zero ethene site-time, rigorously determined initial ethene dimerization rates (453 K, per Ni) that showed a first-order dependence in ethene pressure (0.05-1 kPa). This kinetic dependence implies the β-agostic [Ni(II)-ethyl]+ complex to be the most abundant reactive intermediate for the Beta zeolites containing exchanged and grafted Ni2+ cations. Further, the apparent first-order dimerization rate constant was two orders of magnitude higher for exchanged Ni2+ cations than for grafted Ni2+ cations, reflecting differences in ethene adsorption or dimerization transition state free energies at these two types of Ni sites.
The presence of residual H+ sites on aluminosilicate zeotypes, in addition to the Ni2+ sites, causes formation of saturated hydrocarbons and oligomers that are heavier than butenes and those containing odd numbers of carbon atoms. The reaction pathways on Ni2+ and H+ sites are systematically probed on a model Ni-exchanged Beta catalyst that forms a 1:1 composition of these sites in-situ. The quantitative determination of apparent deactivation orders for the decay of product space-time yields provides insights into the site origins of the products formed. Further, Delplot analysis systematically identifies the primary and secondary products in the reaction network. This strategy shows linear butene isomers to be primary products formed at Ni2+-derived sites, while isobutene is formed as a secondary product by skeletal isomerization at H+ sites. In addition, propene is formed as a secondary product, purportedly by cross-metathesis between linear butene isomers and the reactant ethene at Ni2+-derived sites. Also, ethane is a secondary product that forms by hydrogenation of ethene at H+ sites, with the requisite H2 generated in-situ likely by dehydrogenation and aromatization of ethene at H+ sites.The predominance of the coordination-insertion mechanism at Ni2+-derived sites implies kinetic factors influence isomer distributions within the dimer products, providing an opportunity to influence the selectivity toward linear and terminal alkene products of dimerization. In the case of bifunctional materials, reaction pathways on the Ni2+ and H+ sites dictate the interplay between kinetically-controlled product selectivity at Ni sites and thermodynamic preference of product isomers formed at the H+ sites. In summary, through synthesis of control catalytic materials and rigorous treatment of transient kinetic data, this work presents a detailed mechanistic understanding of the reaction pathways at the Ni2+ and H+ sites, stipulating design parameters that have predictable consequences on the product composition of alkene dimerization and oligomerization.