4 research outputs found
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
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
Fast and Selective Sugar Conversion to Alkyl Lactate and Lactic Acid with Bifunctional CarbonāSilica Catalysts
A novel catalyst design for the conversion of mono- and
disaccharides
to lactic acid and its alkyl esters was developed. The design uses
a mesoporous silica, here represented by MCM-41, which is filled with
a polyaromatic to graphite-like carbon network. The particular structure
of the carbon-silica composite allows the accommodation of a broad
variety of catalytically active functions, useful to attain cascade
reactions, in a readily tunable pore texture. The significance of
a joint action of Lewis and weak BrĆønsted acid sites was studied
here to realize fast and selective sugar conversion. Lewis acidity
is provided by grafting the silica component with SnĀ(IV), while weak
BrĆønsted acidity originates from oxygen-containing functional
groups in the carbon part. The weak BrĆønsted acid content was
varied by changing the amount of carbon loading, the pyrolysis temperature,
and the post-treatment procedure. As both catalytic functions can
be tuned independently, their individual role and optimal balance
can be searched for. It was thus demonstrated for the first time that
the presence of weak BrĆønsted acid sites is crucial in accelerating
the rate-determining (dehydration) reaction, that is, the first step
in the reaction network from triose to lactate. Composite catalysts
with well-balanced Lewis/BrĆønsted acidity are able to convert
the trioses, glyceraldehyde and dihydroxyacetone, quantitatively into
ethyl lactate in ethanol with an order of magnitude higher reaction
rate when compared to the Sn grafted MCM-41 reference catalyst. Interestingly,
the ability to tailor the pore architecture further allows the synthesis
of a variety of amphiphilic alkyl lactates from trioses and long chain
alcohols in moderate to high yields. Finally, direct lactate formation
from hexoses, glucose and fructose, and disaccharides composed thereof,
sucrose, was also attempted. For instance, conversion of sucrose with
the bifunctional composite catalyst yields 45% methyl lactate in methanol
at slightly elevated reaction temperature. The hybrid catalyst proved
to be recyclable in various successive runs when used in alcohol solvent