Single-site Supported Metal Catalysts for Aqueous Phase Conversion of Methane to Oxygenates
Li, Mengwei.
2019
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Throughout history,
natural gas has been both one of the primary sources of energy and the raw material for
chemical production in the industry. Aside from being used as a fuel for power
generation, transportation and heating, natural gas can also be converted to syngas,
which is widely used in industrial processes including production of ammonia, methanol,
dimethyl ether and diesel. The need ... read moreto convert natural gas to syngas as an intermediate
step poses a particular economical challenge as the process is energy intensive and
typically operates at temperatures above 800⁰C. Such a process is only economical
at large scale and difficult to implement for gas resources that are associated with oil
or located at remote areas. An efficient oxidative route to convert natural gas to other
chemicals in one step at low temperature is thus highly desirable. Methane is the main
component of natural gas and has four symmetric C-H bonds with bond energy of 104
kcal/mol. Direct functionalization of methane offers a potential alternative to the
energy demanding syngas route. A one-step oxidative conversion of methane to oxygenates
like methanol and acetic acid is of particular interest since methanol is both a fuel
with high energy density and a feedstock for other chemicals like dimethyl ether, acetic
acid and formaldehyde. Acetic acid, on the other hand, is a high value chemical in large
demand. Its largest use is in the production vinyl acetic followed by solvent use in
terephthalic acid production. This thesis aims to design novel heterogeneous catalysts
for selective oxidation methane to methanol and acetic acid in one-step, with particular
emphasis on the role of supported single atom species for activating and functionalizing
the C-H bond of methane at low temperatures. Rhodium single atom supported on zeolite
was first investigated for the selective oxidation of methane to methanol and acetic
acid. Parametric study on the catalyst was conducted to improve conversion and
selectivity. A low partial pressure of O2 was found to be favorable for the selective
oxidation of methane to acetic acid and the catalyst is catalytic, with turnover number
over 300 in a single batch reaction. The active site of the catalyst was elucidated by
characterization of the catalyst using advanced microscopy and spectroscopy methods and
identified as isolated rhodium cation in intermediate oxidation state anchored inside
the micropores of the zeolite. Furthermore, mechanistic investigation on the reaction
pathway found that single atom of rhodium can activate methane and generate a Rh-CH3
complex and subsequent functionalization follows two parallel pathways: Carbonylation of
the complex yields acetic acid while direct hydroxylation leads to methanol formation.
The coordination environment of the support was found to affect product selectivity.
Support with Brønsted acidity favors the formation of acetic acid, while support
with neutral acidity favors the formation of methanol. Next, iridium-based catalysts was
also studied for this reaction. The monometallic iridium catalyst shows a relatively low
activity due to its tendency to form small clusters and nanoparticles. However, the
addition of a second metal, namely palladium or copper, can significantly promotes both
the activity and selectivity of the catalyst, leading to high methanol yield. The
promoter itself has a negligible activity and there is clearly a synergistic effect
between iridium and the second metal. Detailed studies on the role of each promoter were
performed and the underlying mechanism was presented. Finally, the role of carbon
monoxide in this reaction system is discussed. The presence of CO serves as two roles.
First, it stabilizes rhodium and iridium species in its mononuclear state at reaction
conditions, forming Rh(I)(CO)2 and Ir(I)(CO)2 as characterized by CO-DRIFTS. The ability
of CO to fracture Rh and Ir into single atoms from metallic state can be followed by
CO-DRIFTS. Second, CO molecule serves as the two-electron donor for reductive activation
of molecular oxygen and the generation of peroxo-species. The generation of peroxo
species and its release into aqueous phase can be monitored at temperature as low as
50⁰C but methane oxidation reaction does not light off at temperature below
100⁰C. Low temperature production of H2O2 can be utilized by introducing iron
sites into the zeolite pores and conduct the reaction in acidic
media.
Thesis (Ph.D.)--Tufts University, 2019.
Submitted to the Dept. of Chemical and Biological Engineering.
Advisor: Maria Flytzani-Stephanopoulos.
Committee: Terry Haas, Prashant Deshlahra, Hyunmin Yi, and Yuriy Roman.
Keywords: Chemical engineering, Nanoscience, and Chemistry.read less - ID:
- ms35tn55v
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