24 research outputs found
Branching-First: Synthesizing CāC Skeletal Branched Biobased Chemicals from Sugars
A novel
strategy to biobased chemicals with a branched carbon skeleton
is introduced. Hereto, small sugars, such as 1,3-dihydroxyacetone,
are coupled catalytically to obtain branched C<sub>6</sub> sugars,
such as dendroketose, in high yield at mild conditions. By bringing
this branching step up front, at the level of the sugar feedstock
(<i>branching-first</i>), new opportunities for the synthesis
of useful chemicals arise. Here, we show that the branched sugar can
be efficiently valorized into (i) new branched polyols and (ii) short
branched alkanes. The first route preserves most of the original sugar
functionality by hydrogenation with Ru/C and renders access to branched
polyols with three primary alcohol groups. These molecules are potentially
interesting as plasticizers, cross-linkers, or detergent precursors.
The second valorization route demonstrates a facile hydrodeoxygenation
of the branched sugars toward their corresponding branched alkanes
(e.g., 2-methylpentane). The highest alkanes yields (>65 mol %
C)
are obtained with a Rh/C redox metal catalyst in a biphasic catalytic
system, following a HDO mechanism. In the short term, commercial integration
of these monobranched alkanes, in contrast to branched polyols, is
expected to be straightforward because of their drop-in character
and well-known valuable octane booster role when present in gasoline.
Accordingly, the <i>branching-first</i> concept is also
demonstrated with other small sugars (e.g., tetroses) enabling the
production of branched C<sub>6</sub>āC<sub>8</sub> sugars and
thus also branched C<sub>5</sub>āC<sub>8</sub> alkanes after
HDO
Impact of Hydrotalciteās Basic Sites on the Catalyst Stability and the Branching Selectivity in Ī±āHydroxyketone Aldolization
Reconstructed
hydrotalcites serve as powerful catalysts for the
aldolization of Ī±-hydroxyketones, exemplified here by the glycerol-derived
dihydroxyacetone (DHA), toward the formation of branched hexoses (dendroketoses).
Due to the multichemical functionality of these hydroxyketones, various
competitive reactions such as keto-aldehyde isomerization (e.g., dihydroxyacetone
(DHA)/glyceraldehyde (GLA) equilibrium) are possible, reducing the
branching selectivity of the aldolization reaction. This study reveals
that the nature of the basic sites (as determined via CDCl3 probe FT-IR and CO2-TPD experiments) strongly affects
the branching selectivity of the condensation reaction as well as
the stability of the hydrotalcite catalyst. For instance, strong basic
sites not only allow the undesired keto-aldehyde isomerization but
also promote the Cannizzaro reactions toward the formation of organic
carboxylic acids leading to the catalyst leaching and waste generation.
Thus, subsequent chemical transformations of branched sugars cannot
occur without prior purification. For instance, the hydrogenation
toward biobased branched polyol, which is the industrial target in casu base condensation of DHA. Tuning the basic properties
of the reconstructed hydrotalcite based on this knowledge ultimately
led to an active, selective, and stable catalyst with improved regeneration
possibility
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
Propylphenol to Phenol and Propylene over Acidic Zeolites: Role of Shape Selectivity and Presence of Steam
This
contribution studies the steam-assisted dealkylation of 4-<i>n</i>-propylphenol (4-<i>n</i>-PP), one of the major
products derived from lignin, into phenol and propylene over several
micro- and mesoporous acidic aluminosilicates in gas phase. A series
of acidic zeolites with different topology (<i>e.g</i>.,
FER, TON, MFI, BEA, and FAU) are studied, of which ZSM-5 outperforms
the others. The catalytic results, including zeolite topology and
water stability effects, are rationalized in terms of thermodynamics
and kinetics. A reaction mechanism is proposed by (<i>i</i>) analyzing products distribution under varying temperature and contact
time conditions, (<i>ii</i>) investigating the dealkylation
of different regio- and geometric isomers of propylphenol, and (<i>iii</i>) studying the reverse alkylation of phenol and propylene.
The mechanism accords to the classic carbenium chemistry including
isomerization, disproportionation, transalkylation, and dealkylation,
as the most important reactions. The exceptional selectivity of ZSM-5
is attributed to a pore confinement, avoiding disproportionation/transalkylation
as a result of a transition state shape selectivity. The presence
of water maintains a surprisingly stable catalysis, especially for
ZSM-5 with low acid density. The working hypothesis of this stabilization
is that water precludes diphenyl ether(s) formation in the pores by
reducing the lifetime of the phenolics at the active site due to the
high heat of adsorption of water on H-ZSM-5, besides counteracting
the equilibrium of the phenolics condensation reaction. The water
effect is unique for the combination of (alkyl)Āphenols and ZSM-5
Miniaturized Layer-by-Layer Deposition of MetalāOrganic Framework Coatings through Digital Microfluidics
Miniaturized
Layer-by-Layer Deposition of MetalāOrganic Framework Coatings
through Digital Microfluidic
An Inner-/Outer-Sphere Stabilized Sn Active Site in Ī²āZeolite: Spectroscopic Evidence and Kinetic Consequences
A highly
active Sn site with Lewis acid properties is identified
in post-synthetically synthesized Sn/DeAlĪ² catalyst, prepared
by liquid-phase Sn grafting of a dealuminated Ī²-zeolite. Though
apparently similar Sn active-site structures have been reported for
the post-synthetic and the conventional hydrothermal SnĪ², detailed
study of the electronic structure and redox behavior of Sn with EXAFS,
XANES, DR UVāvis, and TPR clearly reveals dissimilarities in
geometry and electronic properties. A model of the active Sn site
is proposed using a contemporary interpretation of inner-/outer-sphere
coordination, assuming inner-sphere coordination of Sn<sup>IV</sup> with three framework SiO<sup>ā</sup> and one outer-sphere
coordination by a distant charge-balancing SiO<sup>ā</sup>,
resulting in a separated Lewis acidābase pair. Stabilization
of this geometry by a nearby water molecule is proposed. In comparison
with active Sn sites in a hydrothermally synthesized SnĪ², those
in the grafted dealuminated material are sterically less demanding
for substrate approach, while the low inner-sphere coordination of
Sn leads to a stronger Lewis acidity. Proximate silanols in the active-site
pocket, identified by FTIR, <sup>29</sup>Si MAS NMR, <sup>1</sup>Hā<sup>29</sup>Si CP MAS NMR, DR NIR, and TGA, may impact local reagent
concentration and transition states stabilization by hydrogen bonding.
The structural dissimilarity of the active Sn site leads to a different
kinetic behavior. Kinetic experiments using two Lewis-acid-catalyzed
reactions, BaeyerāVilliger and MeerweināPonndorfāVerley,
show differences that are reaction-type dependent and have different
entropic (like sterical demand and hydrogen bonding) and enthalpic
contributions (Lewis acid strength). The active-site model, containing
both inner- and outer-sphere ligands with the zeolite framework, may
be considered as a general model for other grafted Lewis acid single
sites
Influence of Acidic (H<sub>3</sub>PO<sub>4</sub>) and Alkaline (NaOH) Additives on the Catalytic Reductive Fractionation of Lignocellulose
Reductive catalytic fractionation
of lignocellulose is a promising
ālignin-firstā biorefinery strategy wherein lignin is
solvolytically extracted from the cell wall matrix and simultaneously
disassembled, resulting in a stable lignin oil and a solid carbohydrate-rich
residue. Herein, we report on the different influence of acidic (H<sub>3</sub>PO<sub>4</sub>) and alkaline (NaOH) additives on the Pd/C-catalyzed
reductive processing of poplar wood in methanol (MeOH). It was found
that the addition of small quantities of H<sub>3</sub>PO<sub>4</sub> results in three rather than two product streams, since under acidic
conditions both delignification and alcoholysis of hemicellulose are
promoted, leaving behind a cellulose-rich pulp. The simultaneous acid-catalyzed
fractionation of the carbohydrates into separate cellulose and hemicellulose
streams provides opportunities for more efficient downstream conversion,
as processing parameters can be tailored to the needs of both streams.
Alkaline conditions, on the other hand, also enhance delignification,
but additionally cause (i) the formation of lignin products other
than those obtained under neutral and acidic conditions, (ii) a hampered
degree of lignin depolymerization, and (iii) substantial loss of cellulose
from the pulp. Further on, a modified process descriptor (LFFE: <i>lignin first fractionation efficiency</i>) was applied to evaluate
the fractionation efficiency of lignocellulose in its three major
constituents. According to this new efficiency measure, mildly acidic
conditions performed best
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