9 research outputs found
Amine-oxide adsorbents for post-combustion COā capture
Amine functionalized silicas are promising chemisorbent materials for post-combustion COā capture due to the high density of active sites per unit mass of adsorbent that can be obtained by tuning the synthesis protocol, thus resulting in high equilibrium COā adsorption capacities. However, when compared to physisorbents, they have a few disadvantages. Firstly, oxidative degradation of the amine groups reduces the lifetime of these adsorbent materials. Furthermore, rapid heat release following the reaction between amines and COā results in large local temperature spikes which may adversely affect adsorption equilibria and kinetics. Thirdly, there is a lack of fundamental understanding of COā-amine adsorption thermodynamics, which is key to scaling up these materials to an industrial-scale adsorption process. In this dissertation the qualitative and quantitative understanding of these three critical aspects of aminosilica adsorbents have been furthered so these materials can be better evaluated and further tuned as adsorbents for post-combustion COā capture applications.Ph.D
Dynamics of CO<sub>2</sub> Adsorption on Amine Adsorbents. 2. Insights Into Adsorbent Design
Packed bed breakthrough experiments are reported for
commercial
zeolite 13X and 3-aminopropyl-functionalized SBA-15 silica materials
with three different amine loadings. Mass and heat transfer dynamics
for all four materials are modeled successfully. Amine adsorbents
with open pores are found to exhibit faster mass diffusion rates compared
to zeolite 13X. When amine loading is increased by coupling aminopropyl
groups, premature breakthrough combined with a long tail is observed.
Contrary to conventional physisorbants, finite heat losses to the
column wall do not explain the long breakthrough tail. A rate model
that accounts for heterogeneity in diffusion was found to accurately
capture the breakthrough shape of the high loading material. Batch
uptake measurements support the hypothesis that slow diffusion through
the polymer phase is what hampers adsorption kinetics in the high
amine loading adsorbent. The results emphasize the importance of designing
materials that are not overloaded with amine sites, as excessive amine
loadings can lead to depressed adsorption kinetics and premature column
breakthrough
Dynamics of CO<sub>2</sub> Adsorption on Amine Adsorbents. 1. Impact of Heat Effects
The packed bed heat and mass transfer dynamics of CO<sub>2</sub> adsorption onto a 3-aminopropylsilyl-functionalized SBA-15
silica
material are reported. Concentration measurements at the outlet of
the packed bed and temperature profiles inside the bed are measured
simultaneously. Heat and mass transfer models in conjunction with
the linear driving force rate model are used to simulate the concentration
and temperature profiles in the bed. The heat and mass transfer processes
in the amine adsorbent packed bed are successfully captured by the
model, and comparison of isothermal and nonisothermal models reveals
that isothermal models provide an accurate description of the dynamic
mass transport behavior in the adsorption column under the experimental
conditions used in this study. The results help establish that under
certain experimental conditions, heat effects in amine adsorbent packed
beds have a negligible effect on CO<sub>2</sub> breakthrough, and
simple isothermal models can be used to accurately assess adsorption
kinetics
Can the Rate of a Catalytic Turnover Be Altered by Ligands in the Absence of Direct Binding Interactions?
Second
sphere coordination effects ubiquitous in enzymatic catalysis
occur through direct interactions, either covalent or non-covalent,
with reaction intermediates and transition states. We present herein
evidence of indirect second sphere coordination effects in which ligation
of water/alkanols far removed from the primary coordination sphere
of the active site nevertheless alter energetic landscapes within
catalytic redox cycles in the absence of direct physicochemical
interactions with surface species mediating catalytic turnovers. Density
functional theory, in situ X-ray absorption and infrared spectroscopy,
and a wide array of steady-state and transient CO oxidation rate data
suggest that the presence of peripheral water renders oxidation half-cycles
within two-electron redox cycles over Ī¼3-oxo-bridged
trimers in MIL-100(M) more kinetically demanding. Communication between
ligated water and the active site appears to occur through the FeāOāFe
backbone, as inferred from spin density variations on the central
Ī¼3-oxygen ājunctionā. Evidence is provided
for the generality of these second sphere effects in that they influence
different types of redox half-cycles or metals, and can be amplified
or attenuated through choice of coordinating ligand. Specifically
in the case of MIL-100(M) materials, the Cr isostructure can be made
to kinetically mimic the Fe variant by disproportionately hindering
oxidation half-cycles relative to the reduction half-cycles. Kinetic
and spectroscopic inferences presented here significantly expand both
the conceptual definition of second sphere effects as well as the
palette of synthetic levers available for tuning catalytic redox performance
through chemical ligation
Important Roles of Enthalpic and Entropic Contributions to CO<sub>2</sub> Capture from Simulated Flue Gas and Ambient Air Using Mesoporous Silica Grafted Amines
The measurement of
isosteric heats of adsorption of silica supported
amine materials in the low pressure range (0ā0.1 bar) is critical
for understanding the interactions between CO<sub>2</sub> and amine
sites at low coverage and hence to the development of efficient amine
adsorbents for CO<sub>2</sub> capture from flue gas and ambient air.
Heats of adsorption for an array of silica-supported amine materials
are experimentally measured at low coverage using a Calvet calorimeter
equipped with a customized dosing manifold. In a series of 3-aminopropyl-functionalized
silica materials, higher amine densities resulted in higher isosteric
heats of adsorption, clearly showing that the density/proximity of
amine sites can influence the amine efficiency of adsorbents. In a
series of materials with fixed amine loading but different amine types,
strongly basic primary and secondary amine materials are shown to
have essentially identical heats of adsorption near 90 kJ/mol. However,
the adsorption uptakes vary substantially as a function of CO<sub>2</sub> partial pressure for different primary and secondary amines,
demonstrating that entropic contributions to adsorption may play a
key role in adsorption at secondary amine sites, making adsorption
at these sites less efficient at the low coverages that are important
to the direct capture of CO<sub>2</sub> from ambient air. Thus, while
primary amines are confirmed to be the most effective amine types
for CO<sub>2</sub> capture from ambient air, this is not due to enhanced
enthalpic contributions associated with primary amines over secondary
amines, but may be due to unfavorable entropic factors associated
with organization of the second alkyl chain on the secondary amine
during CO<sub>2</sub> adsorption. Given this hypothesis, favorable
entropic factors may be the main reason primary amine based adsorbents
are more effective under air capture conditions
Vision 2050: Reaction Engineering Roadmap
ABSTRACT: This perspective provides the collective opinions of a dozen chemical reaction engineers from academia and industry. In this sequel to the āVision 2020: Reaction Engineering Roadmap,ā published in 2001, we provide our opinions about the field of reaction engineering by addressing the current situation, identifying barriers to progress, and recommending research directions in the context of four industry sectors (basic chemicals, specialty chemicals, pharmaceuticals, and polymers) and five technology areas (reactor system selection, design and scale-up, chemical mechanism development and property estimation, catalysis, nonstandard reactor types, and electrochemical systems). Our collective input in this report includes numerous recommendations regarding research needs in the field of reaction engineering in the coming decades, including guidance for prioritizing efforts in workforce development, measurement science, and computational methods. We see important roles for reaction engineers in the plastics circularity challenge, decarbonization of processes, electrification of chemical reactors, conversion of batch processes to continuous processes, and development of intensified, dynamic reaction processes