<p>The
increase in shale gas exploitation has motivated the studies towards new
processes for converting light alkanes into higher valuable chemicals,
including fuels. The works in this dissertation focuses on two processes:
propane dehydrogenation and ethylene oligomerization. The former involves the
conversion of propane into propylene and hydrogen, while the latter converts
light alkenes into higher molecular weight products, such as butylene and
hexene. </p>
<p>The
thesis project focuses on understanding the effect of geometric effects of Pt
alloy catalysts for propane dehydrogenation and the methodologies for their
characterization. Pt-Co bimetallic catalysts were synthesized with increasing
Co loadings, characterized and evaluated for its propane dehydrogenation
performance. In-situ synchrotron X-Ray Powder Diffraction (XRD) and X-Ray
Absorption (XAS) were used to identify and differentiate between the
intermetallic compound phases in the nanoparticle surface and core. Difference
spectra between oxidized and reduced catalysts suggested that, despite the
increase in Co loading, the catalytic surface remained the same, Pt<sub>3</sub>Co
in a Au<sub>3</sub>Cu structure, while the core became richer in Co, changing
from a monometallic Pt fcc core at the lowest Co loading to a PtCo phase in a
AuCu structure at the highest loading. Co<sup>II</sup> single sites were also
observed on the surface, due to non-reduced Co species. The catalytic
performance towards propane dehydrogenation reinforced this structure, as propylene
selectivity was around 96% for all catalysts, albeit the difference in
composition. The Turnover Rate (TOR) of these catalysts was also similar to
that of monometallic Pt catalysts, around 0.9 s<sup>-1</sup>, suggesting Pt was
the active site, while Co atoms behaved as non-active, despite both atoms being
active in their monometallic counterparts.</p>
<p>In
the second project, a single site Co<sup>II</sup> catalyst supported on SiO<sub>2</sub>
was evaluated for ethylene oligomerization activity. The catalyst was
synthesized, evaluated for propane dehydrogenation, propylene hydrogenation and
ethylene oligomerization activities and characterized <i>in-situ</i> by XAS and EXAFS and H<sub>2</sub>/D<sub>2</sub> exchange
experiments. The catalysts have shown negligible conversion at 250<sup>o</sup>C
for ethylene oligomerization, while a benchmark Ni/SiO<sub>2</sub> catalyst had
about 20% conversion and TOR of 2.3x10<sup>-1</sup> s<sup>-1</sup>. However, as
the temperature increased to above 300<sup>o</sup>C, ethylene conversion
increased significantly, reaching about 98% above 425<sup>o</sup>C. <i>In-situ</i> XANES and EXAFS characterization
suggested that H<sub>2</sub> uptake under pure H<sub>2</sub> increased in about
two-fold from 200<sup>o</sup>C to 500<sup>o</sup>C, due to the loss of
coordination of Co-O bonds and formation of Co-H bonds. This was further
confirmed by H<sub>2</sub>/D<sub>2</sub> experiments with a two-fold increase
in HD formation per mole of Co. <i>In-situ</i>
XAS characterization was also performed with pure C<sub>2</sub>H<sub>4</sub>
at 200<sup>o</sup>C showed a similar trend in Co-O bond loss, suggesting the
formation of Co-alkyl, similarly to that of Co-H. The <i>in-situ</i> XANES spectra showed that the oxidation state remained
stable as a Co<sup>2+</sup> despite the change in the coordination environment,
suggesting that the reactions occurs through a non-redox mechanism. These
combined results allowed the proposition of a reaction pathway for dehydrogenation
and oligomerization reactions, which undergo a similar reaction intermediate, a
Metal-alkyl or Metal-Hydride intermediates, activating C-H bonds at high
temperatures.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/8978828 |
| Date | 16 December 2020 |
| Creators | Laryssa Goncalves Cesar (7022285) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/Light_Alkanes_to_Higher_Molecular_Weight_Olefins_Catalysits_for_Propane_Dehydrogenation_and_Ethylene_Oligomerization/8978828 |
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