31 research outputs found
Au-NHC@Porous Organic Polymers: Synthetic Control and Its Catalytic Application in Alkyne Hydration Reactions
The
synthetic control and functions of porous organic polymers
(POPs) with N-heterocyclic carbene goldĀ(I) (Au-NHC@POPs) are described
in this article. A series of Au-NHC@POPs with tunable physical properties
such as surface area and pore size distribution were first synthesized
via Sonogashira chemistry by differing monomer strut lengths and concentration
during polymerization; a controllable transition from nonporous to
microporous and the coexistence of micro- and mesoporous structures
in the framework were realized by varying the monomer concentration.
To explain this phenomenon, we put forward a model assumption of a
branchābranch cross effect. Additionally, Au-NHC@POPs1 was
found to have superior catalytic activity in alkyne hydration reactions,
and the catalyst could be used six times with a slight loss of activity
Effects of Cellulose, Hemicellulose, and Lignin on the Structure and Morphology of Porous Carbons
Porous
carbon materials stemming from biomass have drawn increasing
interest because of their sustainable properties. Cellulose, hemicellulose,
and lignin are the three basic components of crude biomass, and were
investigated to reveal their influence on the derived carbonaceous
materials. Huge amounts of oxygen-containing functional groups in
cellulose and hemicellulose tend to be eliminated as H<sub>2</sub>O, CO<sub>2</sub>, and CO and give micropores during pyrolysis, whereas
lignin contains plentiful aromatic units which are chemically inert,
and thus produce nonporous carbon materials. When the KHCO<sub>3</sub> was introduced during the pyrolysis process, the plentiful hydroxyl
in cellulose and hemicellulose underwent dehydration condensation
among different parent polymers, which are responsible for the formation
of macroporous structure. By contrast, The Ī²-O-4 bands in lignin
experience homolysis and give rise to benzene-containing units, which
finally result in carbon nanosheets. Furthermore, we demonstrated
the mixture of cellulose, hemicellulose, and lignin can display a
three-dimensional porous structure (containing macropores, mesopores,
and micropores) when less than 50% of lignin is contained
Mapping Out Chemically Similar, Crystallographically Nonequivalent Hydrogen Sites in MetalāOrganic Frameworks by <sup>1</sup>H Solid-State NMR Spectroscopy
Metalāorganic frameworks (MOFs) are important materials
with many actual and potential applications. Crystal structure of
many MOFs is determined by single-crystal X-ray diffraction. However,
due to the inability of XRD to accurately locate hydrogen atoms, the
local structures around framework hydrogen are usually poorly characterized
even if the overall framework has been accurately determined. <sup>1</sup>H solid-state NMR (SSNMR) spectroscopy should, in principle,
be used as a complementary method to XRD for characterizing hydrogen
local environments. However, the spectral resolution of <sup>1</sup>H SSNMR is severely limited by the strong <sup>1</sup>Hā<sup>1</sup>H homonuclear dipolar coupling. In this work, we demonstrate
that high-resolution <sup>1</sup>H MAS spectra of MOF-based material
can be obtained by ultrafast sample spinning at high magnetic field
in combination with isotopic dilution. In particular, we examined
an important MOF, microporous Ī±-Mg<sub>3</sub>(HCOO)<sub>6</sub> and Ī±-Mg<sub>3</sub>(HCOO)<sub>6</sub> in the presence of
several guest species. All six chemically very similar, but crystallographically,
nonequivalent H sites of these MOFs were resolved in a chemical shift
range as small as 0.8 ppm. Although the assignment of <sup>1</sup>H peaks due to crystallographically nonequivalent hydrogens is difficult
due to that they all have almost identical chemical environments,
we are able to show that they can be assigned from <sup>1</sup>Hā<sup>1</sup>H proximity maps obtained from 2D <sup>1</sup>Hā<sup>1</sup>H double quantum (DQ) experiments in conjunction with theoretical
calculations. <sup>1</sup>H MAS spectra of framework hydrogen are
very sensitive to the guest molecules present inside the pores and
they provide insight into hostāguest interaction and dynamics
of guest molecule. The ability of achieving very high resolution for <sup>1</sup>H MAS NMR in MOF-based materials and subsequent spectral assignment
demonstrated in this work allows one to obtain new structural information
complementary to that obtained from single-crystal XRD
Mechanistic Pathways for Methylcyclohexane Hydrogenolysis over Supported Ir Catalysts
H/D
isotope effects on methylcyclohexane hydrogenolysis over Ir/Al<sub>2</sub>O<sub>3</sub> catalysts were examined by combining measured
rates with theoretical estimates provided by partition function based
analyses. Normal H/D isotope effects (<i>r</i><sub>H</sub>/<i>r</i><sub>D</sub> > 1) were observed for endocyclic
and exocyclic CāC bond hydrogenolysis. Hydrogenolysis is concluded
to occur via stepwise dehydrogenation followed by cleavage of the
CāC bond and subsequent hydrogenation of the cleaved entities.
The so-called āmultipletā mechanism (i.e., the CāC
bond of a flat-lying physisorbed cyclic molecule is cleaved upon the
attack of a coadsorbed H atom) is unequivocally excluded. For ring-opening,
either CāC bond cleavage or CāHĀ(D) bond reformation
may be rate-determining, due to their indistinguishable isotope effects
under the studied conditions. CāHĀ(D) bond dissociation does
not control the rate of CāC bond hydrogenolysis. For the exocyclic
cleavage of the methyl group, a higher degree of unsaturation of the
surface intermediate and the potential impact of mobile H atoms on
large Ir particles are noted
Interaction between Histidine and Zn(II) Metal Ions over a Wide pH as Revealed by Solid-State NMR Spectroscopy and DFT Calculations
The interactions between histidine
and metal species play essential
roles in a wide range of important biological processes including
enzymes catalysis and signal transduction. In this work, solid-state
NMR techniques were employed to determine the interaction between
histidine and ZnĀ(II) from pH 3.5 to 14. 2D homo- and heteronuclear
correlation NMR experiments were utilized to extract the <sup>1</sup>H, <sup>13</sup>C, and <sup>15</sup>N chemical shifts in various
histidineāZnĀ(II) binding complexes. Several histidineāZnĀ(II)
binding models were proposed on the basis of experimental results
as well as DFT theoretical calculations. No direct interaction could
be found between biprotonated histidine and ZnĀ(II) at acidic pH. At
pH 7.5, one zinc ion could be hexa-coordinated with two histidine
molecules on Cā², N<sub>Ī±</sub> and deprotonated N<sub>Ī“1</sub> sites. As the pH increases to 11ā14, both of
the N<sub>Ī“1</sub> and N<sub>Īµ2</sub> sites could be deprotonated
as acceptors to be bound to either ZnĀ(II) or water. All of these findings
give a comprehensive set of benchmark values for NMR parameters and
structural geometries in variable histidineāZnĀ(II) binding
complexes over a wide pH range and might provide insights into the
structureāproperty relationship of histidineāmetal complexes
in biological metalloproteins
Stability of the Reaction Intermediates of Ethylbenzene Disproportionation over Medium-Pore Zeolites with Different Framework Topologies: A Theoretical Investigation
The strain energies of the main reaction
intermediates (i.e., monoethylated
diphenylethane (mEDPE) and diethylated diphenylethane (dEDPE) derivatives),
which can be formed during ethylbenzene (EB) disproportionation over
six 10-ring zeolites with different framework topologies, as well
as over the large-pore zeolite Y, were determined by the density functional
theory calculations in order to more precisely investigate the effects
of the pore structure of medium-pore zeolites on their formations.
It was found that while the strain energies of mEDPE and dEDPE intermediates
in zeolite Y, MCM-22 and TNU-9, were always lower than 19.6 kJ mol<sup>ā1</sup>, some of them were characterized by considerably
higher energies (>32.8 kJ mol<sup>ā1</sup>) when positioned
in the intersection channels of ZSM-5 and ZSM-57. As expected, in
addition, all the mEDPE and dEDPE derivatives embedded in TNU-10 and
ZSM-22 with narrower 10-ring channels were strongly distorted, giving
them much higher strain energies (>37.7 kJ mol<sup>ā1</sup>), which were in excellent agreements with our recently reported
experimental results (J.
Phys. Chem. C 2010, 115, 16124). This led us to conclude that the size
and shape of void spaces in the medium-pore zeolites play a crucial
role in governing the type of mEDPE and dEDPE formations during the
EB disproportionation. Our work also shows that the strain energies
of various reaction intermediates confined within zeolites with different
pore topologies could be regarded as a useful quantitative means in
better understanding the shape-selective nature of this important
class of microporous crystalline catalysts
Enhance Hydrogen Isotopes Separation by Alkali Earth Metal Dopant in MetalāOrganic Framework
Kinetic quantum sieving (KQS) based on pore size and
chemical affinity
quantum sieving (CAQS) based on adsorption site are two routes of
porous materials to separate hydrogen isotope mixtures. Alkali earth
metals (Be, Mg, and Ca) were doped into UiO-67 to explore whether
these metal sites can promote H2/D2 separation.
Based on the zero-point energy and adsorption enthalpy calculated
by density functional theory calculations, the Be dopant shows better
H2/D2 separation performance than other alkali
earth metal dopants and unsaturated metal sites in metalāorganic
frameworks based on CAQS. Orbital interaction strongly relates to
the chemical affinity and further influences the D2/H2 selectivity. Moreover, the predicted D2/H2 selectivity of Be-doped sites (49.4) at 77 K is even larger
than the best experimental result (26). Finally, the different dynamic
behaviors of H2 and D2 on Be-doped UiO-67 indicate
its strong H2/D2 separation performance via
KQS
Origin of Zeolite Confinement Revisited by Energy Decomposition Analysis
Our previous work
demonstrated that hydrocarbon species can be stabilized in the confined
zeolite in the form of an ion pair, Ļ complex, and alkoxy species.
Nevertheless, the interaction mechanism between the different reactants/intermediates
and the zeolite framework remains undetermined, and thus, the origin
of the zeolite confinement effect has not been thoroughly revealed.
In this work, a recently developed energy decomposition analysis (EDA)
method was applied to theoretically investigate the energy parameters
of a series of hydrocarbon species confined in the zeolitic catalysts
with different pore diameters. It is demonstrated that for the carbenium
ion intermediates, the electrostatic interaction plays a key role
in their stabilization; for the alkoxy species, both orbital and electrostatic
interactions are the key factors, while for the neutral hydrocarbons,
the dispersion interaction favors their stabilization. In addition,
the principal components analysis (PCA) reveals that the dispersion
interaction does not play a crucial role in improving the reaction
activity due to the same extent of stabilization effect for different
reaction species (e.g., reactant, transition state, intermediate,
or product), and thus, the dispersion contribution would be counteracted
in a specific zeolite catalytic reaction. In contrast, the difference
in electrostatic interaction caused by the variations of charge characteristics
of the various confined species considerably contributes to the decrease
of the activation barrier and the increase of the reaction energy,
which in turn largely promotes the catalytic performance of zeolite
catalysts
Diffusion Dependence of the Dual-Cycle Mechanism for MTO Reaction Inside ZSM-12 and ZSM-22 Zeolites
The
ādual-cycleā pathway (i.e., olefin-based cycle and aromatic-based
cycle) of methanol-to-olefin (MTO) has been generally accepted as
hydrocarbon pool mechanism. Understanding the role of diffusion of
reactant, intermediate, and product in the MTO process is essential
in revealing its reaction mechanism. By using molecular dynamics (MD)
simulations for two one-dimensional zeolites (ZSM-12 and ZSM-22) with
a channel difference being only 0.3 Ć
in pore size, the diffusion
behaviors of some representative species following ādual-cycleā
mechanism (e.g., methanol, polymethylbenzenes, and olefins molecules)
have been theoretically investigated in this work. It was found that
the diffusion coefficients of methanol and olefins along ZSM-12 were
ca. 2ā3 times faster than that along ZSM-22 at 673 K. In the
aromatic-based cycle, the polymethylbenzenes are crucial intermediates
during the MTO reaction. 1,2,3,5-Tetramethylbenzene is almost imprisoned
inside ZSM-12; such slower diffusion of tetramethylbenzene offers
more opportunities for the geminal methylation reaction to form MTO
activated pentamethylbenzenium cation, which would split into olefins
through āparingā or āside-chainā pathways.
However, in the ZSM-22 zeolite, since 1,2,4-trimethylbenzene is stacked,
the following methylation reaction solely results in the formation
of tetramethylbenzene, which is not an MTO activated species in ZSM-22
and more bulky polymethylbenzene further blocks the channel more seriously.
When it comes to the olefin-based cycle, olefins can diffuse freely
inside these two zeolites with methoxide intermediates bound to the
zeolite frameworks, which thus facilitates formation of longer-chain
olefin through olefin methylation reaction in these two zeolite catalysts.
The combination of the higher reaction activity (from DFT calculation)
and the longer contact time (from MD simulation) between the olefin
and methoxide is apparently illustrated as the olefin-based cycle
does more preferentially occur inside ZSM-22 than inside ZSM-12. Apparently,
the MTO reaction mechanism is strongly determined by the diffusion
behaviors of reaction species inside the zeolite confined pores
Solvent Effect Inside the Nanocage of Zeolite Catalysts: A Combined Solid-State NMR Approach and Multiscale Simulation
Solvent effect plays
an important role in manipulating the chemical
reactivity, equilibrium constant, and reaction rate. Such effect is
observed in heterogeneous catalysis, especially for the acidic zeolite
catalyst with molecularly size pores (ā¤1 nm). Nevertheless,
it is a great challenge to systematically investigate the intermolecular
interaction and the mechanism of solvent effect on the catalytic performance
inside the acidic zeolite nanocages. Here, we used the state-of-the-art
solid-state NMR (SSNMR) experimental techniques combined with multiscale
theoretical simulations to quantitatively investigate the solvent
effect on the reactant electronic property and reaction activity.
In particular, a series of <sup>13</sup>C CP/MAS solid-state NMR experiments
with acetone probe for H-ZSM-5 zeolite were performed via changing
the coadsorption amount of nitromethane solvent. It is found that
the solvent effect accounts for the enhancement of the apparent BrĆønsted
acidic strength of zeolite catalysts, and thus promotes the catalytic
reactivity. Furthermore, multiscale theoretical simulations for coabsorption
configurations and electronic properties were employed to elucidate
the mechanism of solvent effect on the zeolite catalysis. Therefore,
so far for the first time the quantitative relationship between solvent
effect and the catalytic performance inside the H-ZSM-5 zeolites has
been established, and the mechanism of solvent effect in nanocage
of zeolites was systematically elucidated