Mechanistic Investigations of Ethene Dimerization and Oligomerization Catalyzed by Nickel-containing Zeotypes

Description

<p>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.</p>

<p> 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 <i>ex-situ</i> 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. <i>In-situ</i> 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 <i>in-situ</i> 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.</p>

<p> 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. </p>

<p> 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 <i>in-situ</i>.
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 <i>in-situ</i> likely by
dehydrogenation and aromatization of ethene at H⁺ sites.</p>

<a>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. </a>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.

Links & Downloads

1. 10.25394/pgs.8378903.v1

Tags

Catalysis and Mechanisms of Reactions

Beta zeolite

nickel

Mechanism

ethene

dimerization

oligomerization

reaction pathway

H/D exchange

coordination-insertion

cossee-arlman

alkene

hydride

Additional Fields

Identifer	oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/8378903
Date	12 October 2021
Creators	Ravi Joshi (6897362)
Source Sets	Purdue University
Detected Language	English
Type	Text, Thesis
Rights	CC BY 4.0
Relation	https://figshare.com/articles/thesis/Mechanistic_Investigations_of_Ethene_Dimerization_and_Oligomerization_Catalyzed_by_Nickel-containing_Zeotypes/8378903