6 research outputs found
Interaction of Biological Molecules with Clay Minerals: A Combined Spectroscopic and Sorption Study of Lysozyme on Saponite
The interaction of hen egg white lysozyme (HEWL) with Na- and Cs-exchanged saponite was investigated using sorption, structural, and spectroscopic methods as a model system to study clayāprotein interactions. HEWL sorption to Na- and Cs-saponite was determined using the bicinchoninic acid (BCA) assay, thermogravimetric analysis, and C and N analysis. For Na-saponite, the TGA and elemental analysis-derived sorption maximum was 600 mg/g corresponding to a surface coverage of 0.85 ng/mm<sup>2</sup> with HEWL occupying 526 m<sup>2</sup>/g based on a cross-sectional area of 13.5 nm<sup>2</sup>/molecule. HEWL sorption on Na-saponite was accompanied by the release of 9.5 Na<sup>+</sup> ions for every molecule of HEWL sorbed consistent with an ion exchange mechanism between the positively charged HEWL (IEP 11) and the negatively charged saponite surface. The <i>d</i>-spacing of the HEWLāNa-saponite complex increased to a value of 4.4 nm consistent with the crystallographic dimensions of HEWL of 3 Ć 3 Ć 4.5 nm. In the case of Cs-saponite, there was no evidence of interlayer sorption; however, sorption of HEWL to the āexternalā surface of Cs-saponite showed a high affinity isotherm. FTIR and Raman analysis of the amide I region of the HEWLāsaponite films prepared from water and D<sub>2</sub>O showed little perturbation to the secondary structure of the protein. The overall hydrophilic nature of the HEWLāNa-saponite complex was determined by water vapor sorption measurements. The clay retained its hydrophilic character with a water content of 18% at high humidity corresponding to 240 H<sub>2</sub>O molecules per molecule of HEWL
Second-Sphere Effects on Methane Hydroxylation in Cu-Zeolites
Two
[Cu<sub>2</sub>O]<sup>2+</sup> cores have been identified as
the active sites of low temperature methane hydroxylation in the zeolite
Cu-MOR. These cores have similar geometric and electronic structures,
yet different reactivity with CH<sub>4</sub>: one reacts with a much
lower activation enthalpy. In the present study, we couple experimental
reactivity and spectroscopy studies to DFT calculations to arrive
at structural models of the Cu-MOR active sites. We find that the
more reactive core is located in a constricted region of the zeolite
lattice. This leads to close van der Waals contact between the substrate
and the zeolite lattice in the vicinity of the active site. The resulting
enthalpy of substrate adsorption drives the subsequent H atom abstraction
stepīøa manifestation of the ānestā effect seen
in hydrocarbon cracking on acid zeolites. This defines a mechanism
to tune the reactivity of metal active sites in microporous materials
Identification of Ī±āFe in High-Silica Zeolites on the Basis of ab Initio Electronic Structure Calculations
Ī±-Fe
is the precursor of the reactive Fe<sup>IV</sup>ī»O core responsible
for methane oxidation in Fe-containing zeolites. To get more insight
into the nature and stability of Ī±-Fe in different zeolites,
the binding of FeĀ(II) at six-membered-ring cation exchange sites (6MR)
in ZSM-5, zeolite beta, and ferrierite was investigated using DFT
and multireference ab initio methods (CASSCF/CASPT2). CASPT2 ligand
field (LF) excitation energies of all sites were compared with the
experimental DR-UVāvis spectra reported by Snyder et al. From
this comparison it is concluded that the 16000 cm<sup>ā1</sup> band of Ī±-Fe, observed in all three zeolites, can uniquely
be assigned to a high-spin square-planar (SP) FeĀ(II) located at a
6MR with an AlāSiāSiāAl sequence, where the Al
atoms are positioned opposite in the ring and as close to each other
as possible. The stability of such conformations is also confirmed
by the binding energies obtained from DFT. The bands at 10000 cm<sup>ā1</sup> in the experimental spectra, assigned to spectator
FeĀ(II), are attributed to six-coordinated trigonal-prismatic FeĀ(II)
species, as calculated for the Ī³-site in ZSM-5. The entatic
effect of the zeolite lattice on the stability of the SP sites was
investigated by making use of the unconstrained FeĀ(II) model complex
FeL<sub>2</sub> (with L = [AlĀ(OH)<sub>4</sub>]<sup>ā</sup>).
The SP conformer is approximately 2 kcal/mol more stable than the
tetrahedral form, indicating that the SP coordination environment
of Ī±-Fe is not imposed by the zeolite lattice but rather electronically
preferred by FeĀ(II) in the environment of four O ligands. A significant
contribution to the stability of the SP conformer is provided by mixing
of the doubly occupied 3d<sub><i>z</i><sup>2</sup></sub> orbital with the higher lying 4s
Magnetic Exchange Coupling in Zeolite Copper Dimers and Its Contribution to Methane Activation
The
highly reactive binuclear [Cu2O]2+ active
site in copper zeolites activates the inert CāH bond of methane
at low temperatures, offering a potential solution to reduce methane
flaring and mitigate atmospheric methane levels. While substantial
progress has been made in understanding the activation of methane
by this core, one critical aspect, the active siteās spin,
has remained undetermined. In this study, we use variable-temperature,
variable-field magnetic circular dichroism spectroscopy to define
the ground state spin of the [Cu2O]2+ active
sites in Cu-CHA and Cu-MFI. This novel approach allows for site-selective
determination of the magnetic exchange coupling between the two copper
centers of specific [Cu2O]2+ cores in a heterogeneous
mixture, circumventing the drawbacks of bulk magnetic techniques.
These experimental findings are coupled to density functional theory
calculations to elucidate magnetostructural correlations in copper
zeolites that are different from those of homogeneous binuclear Cu(II)
complexes. The different spin states for the [Cu2O]2+ cores have different reactivities governed by how methane
approaches the active site. This introduces a new understanding of
zeolite topological control on active site reactivity
Tuning Copper Active Site Composition in Cu-MOR through Co-Cation Modification for Methane Activation
The industrial implementation of a direct methane-to-methanol
process
would lead to environmental and economic benefits. Copper zeolites
successfully execute this reaction at relatively low temperatures,
and mordenite zeolites in particular enable high methanol production.
When loaded to a Cu/Al ratio of 0.45, mordenite (Si/Al 5ā9)
has been shown to host three active sites: two [CuOCu]2+ sites labeled MOR1 and MOR2 and a mononuclear [CuOH]+ site. Also at low copper loadings (Cu/Al < 0.20), mordenite has
been demonstrated to activate methane, but its active site has never
been reported. Here, we investigate Na+ mordenite with
varying copper loadings to better understand copper speciation in
mordenite. At low copper loadings, we uncover an unidentified active
site (āMOR3ā) with a strong overlap with the [CuOH]+ siteās spectroscopic signal. By changing the co-cation,
we selectively speciate more MOR3 relative to [CuOH]+,
allowing its identification as a [CuOCu]2+ site. Active
site identification in heterogeneous catalysts is a frequent problem
due to signal overlap. By changing cation composition, we introduce
an innovative method for simplifying a material to allow better analysis.
This has implications for the study of Cu zeolites for methane-to-methanol
and NOx catalysis, but also for studying
and tuning heterogeneous catalysts in general
Spectroscopic Definition of the Copper Active Sites in Mordenite: Selective Methane Oxidation
Two distinct [CuāOāCu]<sup>2+</sup> sites with methane
monooxygenase activity are identified in the zeolite Cu-MOR, emphasizing
that this CuāOāCu active site geometry, having a ā CuāOāCu
ā¼140Ā°, is particularly formed and stabilized in zeolite
topologies. Whereas in ZSM-5 a similar [CuāOāCu]<sup>2+</sup> active site is located in the intersection of the two 10
membered rings, Cu-MOR provides two distinct local structures, situated
in the 8 membered ring windows of the side pockets. Despite their
structural similarity, as ascertained by electronic absorption and
resonance Raman spectroscopy, the two CuāOāCu active
sites in Cu-MOR clearly show different kinetic behaviors in selective
methane oxidation. This difference in reactivity is too large to be
ascribed to subtle differences in the ground states of the CuāOāCu
sites, indicating the zeolite lattice tunes their reactivity through
second-sphere effects. The MOR lattice is therefore functionally analogous
to the active site pocket of a metalloenzyme, demonstrating that both
the active site and its framework environment contribute to and direct
reactivity in transition metal ion-zeolites