4 research outputs found
Screening Cu-Zeolites for Methane Activation Using Curriculum-Based Training
Machine learning
(ML), when used synergistically with atomistic
simulations, has recently emerged as a powerful tool for accelerated
catalyst discovery. However, the application of these techniques has
been limited by the lack of interpretable and transferable ML models.
In this work, we propose a curriculum-based training (CBT) philosophy
to systematically develop reactive machine learning potentials (rMLPs)
for high-throughput screening of zeolite catalysts. Our CBT approach
combines several different types of calculations to gradually teach
the ML model about the relevant regions of the reactive potential
energy surface. The resulting rMLPs are accurate, transferable, and
interpretable. We further demonstrate the effectiveness of this approach
by exhaustively screening thousands of [CuOCu]2+ sites
across hundreds of Cu-zeolites for the industrially relevant methane
activation reaction. Specifically, this large-scale analysis of the
entire International Zeolite Association (IZA) database identifies
a set of previously unexplored zeolites (i.e., MEI, ATN, EWO, and
CAS) that show the highest ensemble-averaged rates for [CuOCu]2+-catalyzed methane activation. We believe that this CBT philosophy
can be generally applied to other zeolite-catalyzed reactions and,
subsequently, to other types of heterogeneous catalysts. Thus, this
represents an important step toward overcoming the long-standing barriers
within the computational heterogeneous catalysis community
Direct Methane to Methanol: The Selectivity–Conversion Limit and Design Strategies
Currently,
methane is transformed into methanol through the two-step
syngas process, which requires high temperatures and centralized production.
While the slightly exothermic direct partial oxidation of methane
to methanol would be preferable, no such process has been established
despite over a century of research. Generally, this failure has been
attributed to both the high barriers required to activate methane
as well as the higher activity of the CH bonds in methanol compared
to those in methane. However, a precise and general quantification
of the limitations of catalytic direct methane to methanol has yet
to be established. Herein, we present a simple kinetic model to explain
the selectivity–conversion trade-off that hampers continuous
partial oxidation of methane to methanol. For the same kinetic model,
we apply two distinct methods, (1) using ab initio calculations and
(2) fitting to a large experimental database, to fully define the
model parameters. We find that both methods yield strikingly similar
results, namely, that the selectivity of methane to methanol in a
direct, continuous process can be fully described by the methane conversion,
the temperature, and a catalyst-independent difference in methane
and methanol activation free energies, Δ<i>G</i><sup>a</sup>, which is dictated by the relative reactivity of the C–H
bonds in methane and methanol. Stemming from this analysis, we suggest
several design strategies for increasing methanol yields under the
constraint of constant Δ<i>G</i><sup>a</sup>. These
strategies include (1) “collectors”, materials with
strong methanol adsorption potential that can help to lower the partial
pressure of methanol in the gas phase, (2) aqueous reaction conditions,
and/or (3) diffusion-limited systems. By using this simple model to
successfully rationalize a representative library of experimental
studies from the diverse fields of heterogeneous, homogeneous, biological,
and gas-phase methane to methanol catalysis, we underscore the idea
that continuous methane to methanol is generally limited and provide
a framework for understanding and evaluating new catalysts and processes
Direct Methane to Methanol: The Selectivity–Conversion Limit and Design Strategies
Currently,
methane is transformed into methanol through the two-step
syngas process, which requires high temperatures and centralized production.
While the slightly exothermic direct partial oxidation of methane
to methanol would be preferable, no such process has been established
despite over a century of research. Generally, this failure has been
attributed to both the high barriers required to activate methane
as well as the higher activity of the CH bonds in methanol compared
to those in methane. However, a precise and general quantification
of the limitations of catalytic direct methane to methanol has yet
to be established. Herein, we present a simple kinetic model to explain
the selectivity–conversion trade-off that hampers continuous
partial oxidation of methane to methanol. For the same kinetic model,
we apply two distinct methods, (1) using ab initio calculations and
(2) fitting to a large experimental database, to fully define the
model parameters. We find that both methods yield strikingly similar
results, namely, that the selectivity of methane to methanol in a
direct, continuous process can be fully described by the methane conversion,
the temperature, and a catalyst-independent difference in methane
and methanol activation free energies, Δ<i>G</i><sup>a</sup>, which is dictated by the relative reactivity of the C–H
bonds in methane and methanol. Stemming from this analysis, we suggest
several design strategies for increasing methanol yields under the
constraint of constant Δ<i>G</i><sup>a</sup>. These
strategies include (1) “collectors”, materials with
strong methanol adsorption potential that can help to lower the partial
pressure of methanol in the gas phase, (2) aqueous reaction conditions,
and/or (3) diffusion-limited systems. By using this simple model to
successfully rationalize a representative library of experimental
studies from the diverse fields of heterogeneous, homogeneous, biological,
and gas-phase methane to methanol catalysis, we underscore the idea
that continuous methane to methanol is generally limited and provide
a framework for understanding and evaluating new catalysts and processes
Control of Metal–Organic Framework Crystal Topology by Ligand Functionalization: Functionalized HKUST‑1 Derivatives
Metal–organic
framework (MOF) materials are nanoporous crystals
that have attracted intense interest in the fields of adsorption and
catalysis. MOFs are interesting in part because of the concept of
reticular synthesis, which allows families of isostructural crystals
to be developed by varying ligand length and functionality. The predictability
associated with MOF crystal structures is complicated in situations
where ligand functionalization leads to new crystal structures. In
this work, we show experimentally how the crystal structures of derivatives
of HKUST-1 (Cu-BTC) vary for a set of six functionalized ligands:
methyl, ethyl, methoxy, bromo, nitro, and acetamide. Our experiments
indicate that synthesizing MOFs with these functionalized ligands
leads to multiple distinct crystal structures. We further show that
these structures can be rationalized computationally using density
functional theory (DFT) and electron localization function (ELF) calculations.
This analysis led to a simple “design principle” for
predicting the structure using the bonding characteristics of the
functional groups to BTC linkers. This heuristic, in combination with
the more detailed calculations of the kind we have presented, will
be useful in future efforts to predictively control the crystal structure
of similar MOF materials