7 research outputs found
Application Inflation for Internal Medicine Applicants in the Match: Drivers, Consequences, and Potential Solutions
Harvesting Water from Air with High-Capacity, Stable Furan-Based MetalāOrganic Frameworks
We synthesized two isoreticular furan-based metalāorganic frameworks (MOFs), MOF-LA2-1(furan) and MOF-LA2-2(furan) with rod-like secondary building units (SBUs) featuring 1D channels, as sorbents for atmospheric water harvesting (LA = long arm). These aluminum-based MOFs demonstrated a combination of high water uptake and stability, exhibiting working capacities of 0.41 and 0.48 g of water per g of MOF (under isobaric conditions of 1.70 kPa), respectively. Remarkably, both MOFs showed negligible loss in water uptake after 165 adsorption-desorption cycles. These working capacities rival those of MOF-LA2-1(pyrazole), which has a working capacity of 0.55 g of water per g of MOF. The current MOFs stand out for their high water stability as evidenced by 165 cycles of water uptake and release. MOF-LA2-2(furan) is the first aluminum MOF to employ a double \u27long arm\u27 extension strategy, confirmed through single-crystal X-ray diffraction (SCXRD). The MOFs were synthesized using a straightforward synthesis route. This study offers valuable insights into designing durable, water-stable MOFs and underscores their potential for efficient water harvesting
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MOF Linker Extension Strategy for Enhanced Atmospheric Water Harvesting
A linker extension strategy for generating metalāorganic frameworks (MOFs) with superior moisture-capturing properties is presented. Applying this design approach involving experiment and computation results in MOF-LA2-1 {[Al(OH)(PZVDC)], where PZVDC2ā is (E)-5-(2-carboxylatovinyl)-1H-pyrazole-3-carboxylate}, which exhibits an approximately 50% water capacity increase compared to the state-of-the-art water-harvesting material MOF-303. The power of this approach is the increase in pore volume while retaining the ability of the MOF to harvest water in arid environments under long-term uptake and release cycling, as well as affording a reduction in regeneration heat and temperature. Density functional theory calculations and Monte Carlo simulations give detailed insight pertaining to framework structure, water interactions within its pores, and the resulting water sorption isotherm
High-Throughput Experimentation, Theoretical Modeling, and Human Intuition: Lessons Learned in Metal-Organic Framework-Supported Catalyst Design
We have screened an array of 23 metals deposited onto the metalāorganic framework (MOF) NU-1000 for propyne dimerization to hexadienes under different reaction conditions for a total of ~1400 experiments. By a first-of-its-kind study utilizing data-driven algorithms and high-throughput experimentation (HTE) in MOF catalysis, yields on Cu-deposited NU-1000 were improved from 4.2% to 24.4%. Characterization of the most-performant catalysts reveal conversion to hexadiene to be due to the formation of large Cu nanoparticles, which is further supported by reaction mechanisms calculated with density functional theory (DFT). Our results demonstrate both the strengths and weaknesses of the HTE approach. As a strength, HTE excels at being able to find interesting and novel catalytic activity; any a priori theoretical approach would be hard-pressed to find success, as high-performing catalysts required highly specific operating conditions difficult to model theoretically, and initial naĆÆve single-atom models of the active site did not prove representative of the nanoparticle catalysts responsible for conversion to hexadiene. As a weakness, our results show how the HTE approach must be designed and monitored carefully to find success; in our initial campaign (~six months and over half of the total experiments conducted) only minor catalytic performances (up to 4.2% yield) were achieved, which was only improved following a complete overhaul of our HTE approach and questioning our initial assumptions. Thus, the HTE approach is much less automated than it may seem, even if driven by machine learning algorithms ā one must carefully design their HTE campaign to find success
Structure and Site Evolution of Framework Ni Species in MIL-127 MOFs for Propylene Oligomerization Catalysis
A mixed-valence oxotrimer metalāorganic framework
(MOF),
Ni-MIL-127, with a fully coordinated nickel atom and two iron atoms
in the inorganic node, generates a missing linker defect upon thermal
treatment in helium (>473 K) to engender an open coordination site
on nickel which catalyzes propylene oligomerization devoid of any
cocatalysts or initiators. This catalyst is stable for ā¼20
h on stream at 500 kPa and 473 K, unprecedented for this chemistry.
The number of missing linkers on synthesized and activated Ni-MIL-127
MOFs is quantified using temperature-programmed oxidation, 1H nuclear magnetic resonance spectroscopy, and X-ray absorption spectroscopy
to be ā¼0.7 missing linkers per nickel; thus, a majority of
Ni species in the MOF framework catalyze propylene oligomerization.
In situ NO titrations under reaction conditions enumerate ā¼62%
of the nickel atoms as catalytically relevant to validate the defect
density upon thermal treatment. Propylene oligomerization rates on
Ni-MIL-127 measured at steady state have activation energies of 55ā67
kJ molā1 from 448 to 493 K and are first-order in
propylene pressures from 5 to 550 kPa. Density functional theory calculations
on cluster models of Ni-MIL-127 are employed to validate the plausibility
of the missing linker defect and the CosseeāArlman mechanism
for propylene oligomerization through comparisons between apparent
activation energies from steady-state kinetics and computation. This
study illustrates how MOF precatalysts engender defective Ni species
which exhibit reactivity and stability characteristics that are distinct
and can be engineered to improve catalytic activity for olefin oligomerization
Structure and Site Evolution of Framework Ni Species in MIL-127 MOFs for Propylene Oligomerization Catalysis
A mixed-valence oxotrimer metalāorganic framework
(MOF),
Ni-MIL-127, with a fully coordinated nickel atom and two iron atoms
in the inorganic node, generates a missing linker defect upon thermal
treatment in helium (>473 K) to engender an open coordination site
on nickel which catalyzes propylene oligomerization devoid of any
cocatalysts or initiators. This catalyst is stable for ā¼20
h on stream at 500 kPa and 473 K, unprecedented for this chemistry.
The number of missing linkers on synthesized and activated Ni-MIL-127
MOFs is quantified using temperature-programmed oxidation, 1H nuclear magnetic resonance spectroscopy, and X-ray absorption spectroscopy
to be ā¼0.7 missing linkers per nickel; thus, a majority of
Ni species in the MOF framework catalyze propylene oligomerization.
In situ NO titrations under reaction conditions enumerate ā¼62%
of the nickel atoms as catalytically relevant to validate the defect
density upon thermal treatment. Propylene oligomerization rates on
Ni-MIL-127 measured at steady state have activation energies of 55ā67
kJ molā1 from 448 to 493 K and are first-order in
propylene pressures from 5 to 550 kPa. Density functional theory calculations
on cluster models of Ni-MIL-127 are employed to validate the plausibility
of the missing linker defect and the CosseeāArlman mechanism
for propylene oligomerization through comparisons between apparent
activation energies from steady-state kinetics and computation. This
study illustrates how MOF precatalysts engender defective Ni species
which exhibit reactivity and stability characteristics that are distinct
and can be engineered to improve catalytic activity for olefin oligomerization
Influence of 1āButene Adsorption on the Dimerization Activity of Single Metal Cations on UiO-66 Nodes
Grafting metal cations to missing linker defect sites
in zirconium-based
metalāorganic frameworks, such as UiO-66, produces a uniquely
well-defined and homotopic catalytically active site. We present here
the synthesis and characterization of a group of UiO-66-supported
metal catalysts, M-UiO-66 (M = Ni, Co, Cu, and Cr), for the catalytic
dimerization of alkenes. The hydrogenādeuterium exchange via
deuterium oxide adsorption followed by infrared spectroscopy showed
that the last molecular water ligand desorbs from the sites after
evacuation at 300 Ā°C leading to M(OH)-UiO-66 structures. Adsorption
of 1-butene is studied using calorimetry and density functional theory
techniques to characterize the interactions of the alkene with metal
cation sites that are found active for alkene oligomerization. For
the most active Ni-UiO-66, the removal of molecular water from the
active site significantly increases the 1-butene
adsorption enthalpy and almost doubles the catalytic activity for
1-butene dimerization in comparison to the presence of water ligands.
Other M-UiO-66 (M = Co, Cu, and Cr) exhibit 1ā3 orders of magnitude
lower catalytic activities compared to Ni-UiO-66. The catalytic activities
correlate linearly with the Gibbs free energy of 1-butene adsorption.
Density functional theory calculations probing the CosseeāArlman
mechanism for all metals support the differences in activity, providing
a molecular level understanding of the metal site as the active center
for 1-butene dimerization