54 research outputs found
Aqueous Phase Hydroalkylation and Hydrodeoxygenation of Phenol by Dual Functional Catalysts Comprised of Pd/C and H/La-BEA
Aqueous phase catalytic phenol hydroalkylation and hydrodeoxygenation
have been explored using Pd/C combined with zeolite H-BEA and La-BEA
catalysts in the presence of H<sub>2</sub>. The individual steps of
phenol hydrogenation, cyclohexanol dehydration, or alkylation with
phenol were individually investigated to gain insight into the relative
rates in the cascade reactions of phenol hydroalkylation. The hydroalkylation
rate, determined by the concentrations of phenol and cyclohexanol
in phenol hydroalkylation, required the hydrogenation rate to be relatively
slow. The optimized H<sup>+</sup>/Pd ratio was 21, which allowed achieving
comparable cyclohexanol formation rates via phenol hydrogenation and
consumption rates from alkylation with phenol in phenol hydroalkylation.
La-BEA was shown to be more selective for hydroalkylation than H-BEA
in combination with Pd/C, because cyclohexanol dehydration was retarded
selectively compared to alkylation of phenol. This indicates that
dehydration is solely catalyzed by BrĆønsted acid sites, while
alkylation can be achieved in the presence of La<sup>3+</sup> cations
Dynamic Phase Separation in Supported PdāAu Catalysts
SiO<sub>2</sub>-supported PdāAu catalysts with Pd/Au molar
ratios varying from 0.8 to 7.0 were used as catalysts for vinyl acetate
synthesis under industrial conditions. Continued operation of the
bimetallic catalysts at 150 Ā°C led to the formation of the Pd<sub>1</sub>Au<sub>1</sub> phase in the particles, with the remaining
Pd atoms forming Pd nanoparticles by leaching of Pd as acetate. The
presence of these phases was monitored by X-ray absorption spectroscopy
(XAS) of the used catalysts. Temperature-resolved in situ XRD of the
reduced samples in an inert atmosphere confirmed the phase separation
into a Pd-rich phase and a Au-rich phase above 160 Ā°C. CO adsorption
and XRD of the catalysts used at 180 Ā°C showed that phase separation
also took place during vinyl acetate synthesis. The pronounced temperature
dependence of the morphology and surface composition of the bimetallic
PdāAu catalysts determines the selectivity; the activity; and,
in particular, the stability during vinyl acetate synthesis
Ni-Catalyzed Cleavage of Aryl Ethers in the Aqueous Phase
A novel Ni/SiO<sub>2</sub>-catalyzed route for selective
cleavage
of ether bonds of (lignin-derived) aromatic ethers and hydrogenation
of the oxygen-containing intermediates at 120 Ā°C in presence
of 6 bar H<sub>2</sub> in the aqueous phase is reported. The CāO
bonds of Ī±-O-4 and Ī²-O-4 linkages are cleaved by hydrogenolysis
on Ni, while the CāO bond of the 4-O-5 linkage is cleaved via
parallel hydrogenolysis and hydrolysis. The difference is attributed
to the fact that the C<sub>aliphatic</sub>āOH fragments generated
from hydrolysis of Ī±-O-4 and Ī²-O-4 linkages can undergo
further hydrogenolysis, while phenol (produced by hydrolysis of the
4-O-5 linkage) is hydrogenated to produce cyclohexanol under conditions
investigated. The apparent activation energies, <i>E</i><sub>a</sub>(Ī±-O-4) < <i>E</i><sub>a</sub>(Ī²-O-4)
< <i>E</i><sub>a</sub>(4-O-5), vary proportionally with
the bond dissociation energies. In the conversion of Ī²-O-4 and
4-O-5 ether bonds, CāO bond cleavage is the rate-determining
step, with the reactants competing with hydrogen for active sites,
leading to a maximum reaction rate as a function of the H<sub>2</sub> pressure. For the very fast CāO bond cleavage of the Ī±-O-4
linkage, increasing the H<sub>2</sub> pressure increases the rate-determining
product desorption under the conditions tested
Atomistic Engineering of Catalyst Precursors: Dynamic Reordering of PdAu Nanoparticles during Vinyl Acetate Synthesis Enhanced by Potassium Acetate
The presence of potassium acetate
(KOAc) on bimetallic PdAu catalysts increases the rate of reaction
for vinyl acetate (VA) formation from ethene and acetic acid by a
factor of 10 and the selectivity by 20%. The dynamic transitions of
typical supported catalyst precursors with an atomic Pd/Au ratio of
2/1 were explored during synthesis in the presence and absence of
KOAc. The dopant induces reordering of PdAu toward a Pd<sub>1</sub>Au<sub>1</sub> phase, while Au-enriched Pd<sub>40</sub>Au<sub>60</sub> bimetallic particles form primarily in the absence of KOAc. Pdāacetate
species are generated via leaching of Pd from PdAu precursor particles
during the reaction. These species are Pd<sub>3</sub>(OAc)<sub>6</sub> and Pd<sub>2</sub>(OAc)<sub>4</sub> in the absence of KOAc and K<sub>2</sub>Pd<sub>2</sub>(OAc)<sub>6</sub> in the presence of KOAc. Palladium
in K<sub>2</sub>Pd<sub>2</sub>(OAc)<sub>6</sub> can be readily reduced
by C<sub>2</sub>H<sub>4</sub> to Pd<sup>0</sup>, while Pd<sub>3</sub>(OAc)<sub>6</sub>, which contains more stable, bridged acetate ligands
remains stable. Reduced Pd either forms dispersed Pd<sup>0</sup> or
is incorporated into the metal particles. KOAc enhances rates and
selectivity to VA by stabilizing, on the one hand, active Pd species
at the bimetallic surface. On the other hand, KOAc enriches acetic
acid close to the surface and forms Pd surface acetates, postulated
to enhance the rate and the selectivity to VA by suppressing ethylene
adsorption and oxidation
Oxidative Dehydrogenation of Ethane on Dynamically Rearranging Supported Chloride Catalysts
Ethane
is oxidatively dehydrogenated with a selectivity up to 95%
on catalysts comprising a mixed molten alkali chloride supported on
a mildly redox-active Dy<sub>2</sub>O<sub>3</sub>-doped MgO. The reactive
oxyanionic OCl<sup>ā</sup> species acting as active sites are
catalytically formed by oxidation of Cl<sup>ā</sup> at the
MgO surface. Under reaction conditions this site is regenerated by
O<sub>2</sub>, dissolving first in the alkali chloride melt, and in
the second step dissociating and replenishing the oxygen vacancies
on MgO. The oxyanion reactively dehydrogenates ethane at the meltāgas
phase interface with nearly ideal selectivity. Thus, the reaction
is concluded to proceed via two coupled steps following a Mars-van-Krevelen-mechanism
at the solidāliquid and gasāliquid interface. The dissociation
of O<sub>2</sub> and/or the oxidation of Cl<sup>ā</sup> at
the meltāsolid interface is concluded to have the lowest forward
rate constants. The compositions of the oxide core and the molten
chloride shell control the catalytic activity via the redox potential
of the metal oxide and of the OCl<sup>ā</sup>. Traces of water
may be present in the molten chloride under reaction conditions, but
the specific impact of this water is not obvious at present. The spatial
separation of oxygen and ethane activation sites and the dynamic rearrangement
of the surface anions and cations, preventing the exposure of coordinatively
unsaturated cations, are concluded to be the origin of the surprisingly
high olefin selectivity
Stabilizing Catalytic Pathways via Redundancy: Selective Reduction of Microalgae Oil to Alkanes
A new route to convert crude microalgae oils using ZrO<sub>2</sub>-promoted Ni catalysts into diesel-range alkanes in a cascade
reaction
is presented. Ni nanoparticles catalyze the selective cleavage of
the CāO of fatty acid esters, leading to the hydrogenolysis
of triglycerides. Hydrogenation of the resulting fatty acids to aldehydes
(rate-determining step) is uniquely catalyzed via two parallel pathways,
one via aldehyde formation on metallic Ni and the second via a synergistic
action by Ni and ZrO<sub>2</sub> through adsorbing the carboxylic
groups at the oxygen vacancies of ZrO<sub>2</sub> to form carboxylates
and subsequently abstracting the Ī±-hydrogen atom to produce
ketene, which is in turn hydrogenated to aldehydes and decarbonylated
on Ni nanoparticles
Formation of CO<sub>2</sub> and Ethane from Propionyl over Platinum: A Density Functional Theory Study
The
conversion of propionyl, the most stable dehydrogenation product
of 1-propanol on flat and stepped Pt(111) surfaces is explored computationally
as a model for the transformation of alcohols on Pt to CO<sub>2</sub> and hydrocarbons. We studied a reaction network in which dehydrogenation
steps or hydroxyl insertion may precede the desired CāC cleavage,
via either decarboxylation or decarbonylation. In the latter case,
CO<sub>2</sub> will be obtained via a subsequent water-gas-shift reaction.
On a flat surface, the decarbonylation pathway with the highest barrier
of 91 kJ mol<sup>ā1</sup> was calculated to be preferred, as
the decarboxylation reactions are inaccessible because of high barriers
of the preceding dehydrogenations, 129 kJ mol<sup>ā1</sup>,
or the CāC bond scission itself, 204 kJ mol<sup>ā1</sup>. At step defects the highest barrier for decarboxylation was determined
to be notably lower, at 86 kJ mol<sup>ā1</sup>, making this
pathway competitive for CāC scission
Mechanism and Kinetics of CO<sub>2</sub> Adsorption on Surface Bonded Amines
The impact of H<sub>2</sub>O on the
mechanism and kinetics of CO<sub>2</sub> adsorption by amine-impregnated
SBA-15 has been investigated.
The H<sub>2</sub>O-free adsorption mechanism proceeds predominantly
via formation of carbamates stabilized between two amine groups. Bicarbonates
and surface stabilized carbamates were formed on hydrated sorbents.
Film diffusion limitations were not observed during adsorption of
CO<sub>2</sub> at high amine loadings. Gaseous water decreased the
adsorption rate but increased the maximum equilibrium uptake as well
as the uptake before breakthrough of the reactor bed. A water film
is formed on the adsorbent particles in gas streams containing more
than 5 vol %, gaseous H<sub>2</sub>O limiting the interaction of CO<sub>2</sub> with the active amine sites and constraining the rates of
adsorption. The higher uptake of CO<sub>2</sub> in the presence of
H<sub>2</sub>O vapor is a result of a change in the adsorption mechanism
that increases the amine efficiency, thus leading to a higher adsorption
capacity. The adsorption was fully reversible for all adsorbents at
a maximum desorption temperature of 100 Ā°C
Characterization of Fe-Exchanged BEA Zeolite Under NH<sub>3</sub> Selective Catalytic Reduction Conditions
Structural changes to Fe<sup>3+</sup> cationic species
in Fe-exchanged
zeolite BEA during the selective catalytic reduction (SCR) of NO<sub><i>x</i></sub> with NH<sub>3</sub> were probed by UVāvis
spectroscopy. The distribution between Fe<sup>2+</sup> and Fe<sup>3+</sup> species was characterized by IR spectroscopy of adsorbed
CO. Upon heating to 723 K, some of the Fe cations formed FeāOāFe
bonds that underwent reversible structural transformation under NH<sub>3</sub>āSCR conditions. The in situ formed Fe oxide clusters
could be dissociated to isolated Fe cations at 423 K, while at higher
temperatures O-bridged Fe clusters were again formed. The structure
of the Fe cluster is related to the Al distribution in the zeolite
probed by Co<sup>2+</sup> ion exchange. We propose here that two Fe
cations bound within one six-membered ring containing an Al pair form
hydroxylated dimeric FeāOāFe in the zeolite. This was
supported by a structure simulation of a binuclear [HO-FeĀ(III)āOāFeĀ(III)āOH]
model
Pathways for H<sub>2</sub> Activation on (Ni)-MoS<sub>2</sub> Catalysts
The activation of H<sub>2</sub> and
H<sub>2</sub>S on (Ni)ĀMoS<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub> leads
to the formation of SH
groups with acid character able to protonate 2,6-dimethylpyridine.
The variation in concentrations of SH groups induced by H<sub>2</sub> and H<sub>2</sub>S adsorption shows that both molecules dissociate
on coordinatively unsaturated cations and neighboring S<sup>2ā</sup>. In the studied materials, one sulfur vacancy and four SH groups
per 10 metal atoms exist at the active edges of MoS<sub>2</sub> under
the conditions studied. H<sub>2</sub>āD<sub>2</sub> exchange
studies show that Ni increases the concentration of active surface
hydrogen by up to 30% at the optimum Ni loading, by increasing the
concentration of H<sub>2</sub> and H<sub>2</sub>S chemisorption sites
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