14 research outputs found
Parahydrogen-Induced Polarization Detected with Continuous Flow Magic Angle Spinning NMR
Parahydrogen-induced polarization (PHIP) detected as
an enhancement
of NMR signal at pairwise addition of hydrogen atoms from the H<sub>2</sub> molecule to olefin is an effective tool for discriminating
among the possible mechanisms of hydrogenation reaction. Previously
reported PHIP effects for heterogeneous hydrogenation were observed
only for gas-phase molecules, that is, for those desorbed from the
catalyst surface. In this case, valuable information on the possible
intermediates and the reaction products on the catalyst surface could
not be provided. In this paper, a magic angle spinning (MAS) NMR technique
was applied for the first time for registration of PHIP produced in
hydrogenation of propene with para-H<sub>2</sub> on the Pt-supported
on metal oxide catalyst (Pt/TiO<sub>2</sub>). We demonstrate that
the use of the <sup>1</sup>H MAS NMR technique under the condition
of a continuous flow of the reagents (CF MAS NMR) enables registration
of polarization in both the gas phase and the adsorbed species. Application
of CF MAS NMR could be useful for detecting PHIP effects on heterogeneous
catalysts and for the study of the mechanisms of heterogeneously catalyzed
hydrogenation reactions
Characterization of Fast Restricted Librations of Terephthalate Linkers in MOF UiO-66(Zr) by <sup>2</sup>H NMR SpināLattice Relaxation Analysis
<sup>2</sup>H NMR spinālattice relaxation was used to probe
small-amplitude torsional vibrations (librations) of the organic terephthalate
linkers in metalāorganic framework (MOF) UiO-66Ā(Zr) saturated
with benzene molecules. In UiO-66 (Zr) the mobile phenylene fragments
exhibit a complex rotational dynamics of the phenylene rings with
fast librations and much slower Ļ-flips around the <i>C</i><sub>2</sub> symmetry axis. We show that due to the intrinsic broad
distribution of the Ļ-flips rate, the relaxation process for
the deuterium in the CāD group of phenylene fragment is multiexponential.
Two main modes of <i>T</i><sub>1</sub> relaxation are clearly
detected, corresponding to the fast <i>T</i><sub>1</sub><sup>fast</sup> and the slow <i>T</i><sub>1</sub><sup>slow</sup> relaxation. Based on the experimental observation of two-exponential
relaxation, a computational model for this <i>T</i><sub>1</sub> relaxation behavior capable to reproduce the peculiarities
of the MOF linkers dynamics was built. Computational analysis allows
to establish that the librational motion affects mostly the <i>T</i><sub>1</sub><sup>slow</sup>, while <i>T</i><sub>1</sub><sup>fast</sup> remains unaffected by this motion. Simulation
of the <i>T</i><sub>1</sub><sup>slow</sup> dependence on
the libration rate <i>k</i><sub>lib</sub> shows that in
the range of the librational frequencies of 10<sup>6</sup>ā10<sup>9</sup> Hz the <i>T</i><sub>1</sub><sup>slow</sup> is not
sensitive to the <i>k</i><sub>lib</sub> variation, and therefore
a precise correspondence between <i>T</i><sub>1</sub><sup>slow</sup> and <i>k</i><sub>lib</sub> cannot be established. <i>T</i><sub>1</sub><sup>slow</sup> exhibits a specific āpeak-like-shapeā
dependence of <i>k</i><sub>lib</sub> in the range of 10<sup>9</sup>ā10<sup>12</sup> Hz. In this range of libration frequencies
an unambiguous relation between <i>T</i><sub>1</sub><sup>slow</sup> and <i>k</i><sub>lib</sub> exists only in a
very narrow frequency window of 0.1 Ć 10<sup>10</sup>ā5
Ć 10<sup>10</sup> Hz. The best conditions to characterize the
librational motion by means of <i>T</i><sub>1</sub> relaxation
analysis are met when the flipping motion is almost frozen (<i>k</i><sub>flip</sub> < 10<sup>3</sup> Hz) because <i>T</i><sub>1</sub><sup>slow</sup> becomes extremely sensitive
to the variation of <i>k</i><sub>lib</sub>
Guest Controlled Rotational Dynamics of Terephthalate Phenylenes in MetalāOrganic Framework MIL-53(Al): Effect of Different Xylene Loadings
MIL-53 is an interesting metalāorganic
framework (MOF) with a ābreathingā framework which is
envisioned for a number of potential applications. It is suggested
that the processes of hydrocarbon adsorption, diffusion, and separation
by this material are strongly influenced by flexibility of the framework
and fastly moving terephthalate phenylene fragments, representing
the array of molecular rotors. To govern the mentioned processes we
need to learn how to monitor the flexibility of the framework and
identify the specific effects of particular hydrocarbons on the rotational
motion of phenylene fragments. Here we demonstrate that flexibility,
i.e., large pore (LP) and narrow pore (NP) crystalline state interconversions
of the framework, can be monitored by following the evolution of phenylene
fragments dynamics with temperature by <sup>2</sup>H solid-state nuclear
magnetic resonance. We have established that the dynamics of phenylene
fragments is very sensitive to the loading of xylene guests and the
MOF structural state. The rotation rate is higher and the activation
barrier lower for the LP state of the guest-free or loosely loaded
material, whereas the NP and LP states with high loadings and dense
guest packing show a decrease of the rotation rates and increase of
the rotation energetic barrier
Mobility of the 2āMethylimidazolate Linkers in ZIFā8 Probed by <sup>2</sup>H NMR: Saloon Doors for the Guests
ZIF-8
is one of the most interesting metalāorganic frameworks
due to its high stability and unique capabilities for hydrocarbons
separation. Its porous network is formed by large almost spherical
cavities interconnected by very narrow windows, ā¼3.4 Ć
,
which should be too small even for methane. At the same time the direct
experimental observations show that ZIF-8 cavities are able to host
even such large molecules as benzene. This effect is associated with
the flexibility of the cavity widows, arising from dynamical freedom
on the 2-methylimidazole (2-mIM) linkers that form the framework.
In this work, by means of <sup>2</sup>H NMR we show that the 2-mIM
linkers of the ZIF-8 are very mobile and their mobility is sensitive
to the presence of benzene guest. In contrast with other known MOFs
based on linearly bonded carboxylates, in guest-free ZIF-8 the plane
of 2-mIM linker exhibits two-site flips within a sector of 2Ļ<sub>f</sub> = 34Ā° with very low activation barrier (1.5 kJ mol<sup>ā1</sup>) and high rates (ā¼10<sup>12</sup> s<sup>ā1</sup>). Above 380 K the linkers begin to demonstrate additional fast librations
with gradually increasing amplitudes Ī³<sub>lib</sub> comparable
with the two-site flips (Ī³<sub>lib</sub> = Ā±17Ā° above
560 K). This is direct evidence that the ZIF-8 linkers twist notably,
thus increasing the aperture of the windows sufficiently to fit very
large molecules. Upon benzene loading, the geometry of the observed
motions remains similar but the potential barrier of the linkers flipping
rises up to 9 kJ mol<sup>ā1</sup>
Mobility of <i>tert-</i>Butyl Alcohol in MFI Framework Type Studied by Deuterium NMR
The molecular mobility of deuterated <i>tert-</i>butyl
alcohol (TBA) adsorbed in MFI framework type (silicalite-1 and ZSM-5
zeolite) has been studied by use of <sup>2</sup>H NMR spectroscopy
in the range of 106ā453 K. In H-ZSM-5, the reorientation of
the molecule as a whole is strongly restricted (Ļ<sub>C</sub> āŖ <i>Q</i><sub>0</sub><sup>ā1</sup> ā
10<sup>ā6</sup> s) by hydrogen bonding to BrĆønsted acid
sites (BAS). Being adsorbed to BAS, the motion of TBA molecules is
described by intramolecular rotations around two successive <i>C</i><sub>3</sub> and <i>C</i><sub>3</sub>ā²
axes (CD<sub>3</sub>āC and CāO bonds). The activation
energy for the methyl groups rotation around the CāO bond (<i>E</i><sub>a</sub> = 8.0 Ā± 1.6 kJ mol<sup>ā1</sup>) is two times lower compared to that in solid TBA (<i>J. Phys.
Chem. A</i> <b>2011</b>, <i>115</i>, 7428). This
shows that the motion of the butyl fragment of TBA in MFI framework
is less restricted compared to the case of solid TBA. In silicalite-1,
the TBA molecule is additionally involved into reorientational motions
as a whole: one of the motions represents an exchange among orientations
provided by the directions of four framework channels driven by the
translational jump diffusion. The activation barriers for the jump
diffusion between two zigzag channels (<i>E</i><sub>a</sub> = 7 Ā± 2 kJ mol<sup>ā1</sup>) and between zigzag and
straight channels (<i>E</i><sub>a</sub> = 5 Ā± 1 kJ
mol<sup>ā1</sup>) are of the similar values. The other motion
represents a large-amplitude wobbling of the TBA molecule localized
at a channel intersection site cavity. This motion is described by
a fast restricted wobbling of the molecular axis in a sphere sector.
The wobbling boundaries become gradually broader as temperature increases,
allowing thus a larger accessible space for the TBA molecule to explore.
The anisotropy of this motion persists even at 453 K, revealing the
presence of a strong intracavity barrier that blocks the TBA from
free rotational diffusion inside the cavity. This study demonstrates
that the <sup>2</sup>H NMR analysis of both line shape and spinālattice
relaxation represents a powerful tool to investigate the particular
pore confinement effect on the molecular mobility of TBA adsorbed
in the MFI framework
Rotational and Translational Motion of Benzene in ZIFā8 Studied by <sup>2</sup>H NMR: Estimation of Microscopic Self-Diffusivity and Its Comparison with Macroscopic Measurements
In relation to unique properties
of metalāorganic framework
(MOF) ZIF-8 to adsorb and separate hydrocarbons with kinetic diameters
notably larger than the entrance windows of the porous system of this
microporous material, the molecular dynamics of benzene adsorbed on
ZIF-8 has been characterized and quantified with <sup>2</sup>H nuclear
magnetic resonance. We have established that within the ZIF-8 cage
the benzene molecule undergoes fast rotations, hovering in the symmetric
potential of the spherical cage and relatively slow isotropic reorientations
by collisions with the walls. Benzene performs also translational
jump diffusion between neighboring cages characterized by an activation
barrier <i>E</i><sub>D</sub> = 38 kJ mol<sup>ā1</sup> and a pre-exponential factor Ļ<sub>D0</sub> = 4 Ć 10<sup>ā10</sup> s. This microscopic measurement of benzene mobility
allows us to estimate the self-diffusion coefficient for benzene in
ZIF-8 (<i>D</i><sup>0</sup><sub>self</sub> ā 4 Ć
10<sup>ā16</sup> m<sup>2</sup> s<sup>ā1</sup> at <i>T</i> = 323 K). Macroscopic measurements of diffusivities derived
from membrane permeation studies (3.5 Ć 10<sup>ā15</sup> m<sup>2</sup> s<sup>ā1</sup> at <i>T</i> = 298
K for fractional occupancy Ī ā 0.99) and sorption uptake
(<i>D</i><sub>i,MS</sub> ā 10<sup>ā20</sup> m<sup>2</sup> s<sup>ā1</sup> at 323 K) are several orders
of magnitude larger or smaller than the microscopic self-diffusion
coefficient <i>D</i><sup>0</sup><sub>self</sub>, which was
derived from relaxation time analysis. This experimental finding is
attributed to the limits of macroscopic measurements
Probing the Guest-Mediated Structural Mobility in the UiO-66(Zr) Framework by <sup>2</sup>H NMR Spectroscopy
The
solid-state <sup>2</sup>H NMR technique (analysis of both the
spectrum line shape and the spinālattice relaxation) was used
to probe both slow and fast dynamical modes of the phenylene fragments
of terephthalate linkers of the UiO-66Ā(Zr) framework affected by the
presence of benzene guest in the pores of the material. Such approach
allowed us to probe different motions within a broad range of time
scale, 10<sup>ā3</sup>ā10<sup>ā11</sup> s. The
internal dynamics in the UiO-66Ā(Zr) framework is represented by torsional
motions of the phenylene fragment of the linker including 2-site 180Ā°
flips (Ļ-flips) of the plane of the phenylene ring and its restricted
librations. In the presence of benzene loaded in the MOF pores the
rate of Ļ-flips decreases essentially and the activation barrier
for this motion increases. The activation barrier has been found to
increase almost in a linear fashion on benzene loading. Such observation
is surprisingly unique among other MOFs with mobile linkers, like
MIL-53Ā(Al) or MOF-5. The fast librational motion occurs on a scale
of ā¼10<sup>10</sup> Hz and shows no notable dependence on the
guest loading. It has been established that anisotropy of <i>T</i><sub>1</sub> relaxation of the <sup>2</sup>H NMR powder
pattern of the phenylene fragments is especially sensitive to the
librational motion when this motion is in a range of 10<sup>7</sup>ā10<sup>11</sup> Hz. Within this range of libration frequencies,
analysis of the anisotropic spinālattice (<i>T</i><sub>1</sub>) relaxation allows quantitative estimation of the rate
of librational motion
Methane Activation on In-Modified ZSMā5 Zeolite. H/D Hydrogen Exchange of the Alkane with BrĆønsted Acid Sites
In
relation to clarifying the pathway of methane activation on
In-modified zeolites, a comparative analysis of kinetics of hydrogen
(H/D) exchange between methane-<i>d</i><sub>4</sub> and
BrĆønsted acid sites (BAS) for both the pure acid form zeolite
(H-ZSM-5) and In-modified zeolites (In<sup>+</sup>/H-ZSM-5 and InO<sup>+</sup>/H-ZSM-5) has been performed. Monitoring of the kinetics has
been carried out with <sup>1</sup>H magic-angle spinning NMR spectroscopy
in situ within the temperature range of 453ā568 K. While the
rate of exchange on In<sup>+</sup>/H-ZSM-5 is 1 order of magnitude
larger than that on H-ZSM-5, the exchange occurs on InO<sup>+</sup>/H-ZSM-5 by 2 orders of magnitude faster than that on H-ZSM-5. Significant
increase of the rate and decrease of the activation energy (<i>E</i><sub>a</sub> = 74 Ā± 6 kJāmol<sup>ā1</sup>) and the temperature threshold (453 K) for the reaction of the exchange
on InO<sup>+</sup>/H-ZSM-5 compared to the rate, activation energy,
and temperature threshold (543 K) for the reaction on In<sup>+</sup>/H-ZSM-5 (<i>E</i><sub>a</sub> = 127 Ā± 27 kJāmol<sup>ā1</sup>) and H-ZSM-5 (<i>E</i><sub>a</sub> = 118
Ā± 9 kJāmol<sup>ā1</sup>) have been rationalized
in terms of involvement of both InO<sup>+</sup> and BAS in activation
of methane molecules on the zeolite. Some transient intermediate complex
of methane with the zeolite InO<sup>+</sup> species and BAS has been
assumed to be formed within the zeolite pore. This complex is involved
either in the reaction of H/D exchange with BAS of the zeolite or
evolves further to offer indium methyl species
Characterization and Dynamics of the Different Protonic Species in Hydrated 12-Tungstophosphoric Acid Studied by <sup>2</sup>H NMR
Solid
heteropolyacids (HPAs) are promising nonpolluting superacids
used as catalysts and solid proton conductors. The catalytic and conducting
properties of HPAs are very sensitive to the amount of hydration water
present in the system, as water molecules tend to interact with the
HPA acid protons to form [H<sub>3</sub>O]<sup>+</sup> and [H<sub>5</sub>O<sub>2</sub>]<sup>+</sup> ions. These ions constitute active species
that govern the catalytic reaction pathways and the proton migration
mechanism. Establishing the structure and mobility of protonic species
could yield important information concerning the functions of material
based on HPA hydrates. In this work, we have performed the analysis
of both <sup>2</sup>H NMR line shape and the evolution of <i>T</i><sub>1</sub>, <i>T</i><sub>2</sub> relaxation
with temperature for the deuterated analogue of the solid 12-tungstophosphoric
acid (TPA Ć <i>n</i>H<sub>2</sub>O) at different hydration
levels (0 < <i>n</i> < 6) in the wide temperature
range of 103ā503 K. This allowed us to characterize in detail
the mobility of different protonic species, including acidic OH groups,
water molecules, and hydroxonium ions. Kinetic parameters of internal
and diffusional motions for different protonic species at different
hydration levels <i>n</i> of TPA Ć <i>n</i>H<sub>2</sub>O were derived
Methane Activation on Zn<sup>2+</sup>-Exchanged ZSMā5 Zeolites. The Effect of Molecular Oxygen Addition
In relation to the reported methane
activation on Zn-modified zeolite
ZSM-5 at room temperature to afford the surface methoxy species by
Xu et al. (<i>Chem. Sci.</i> <b>2012</b>, <i>3</i>, 2932), the activation of methane on Zn<sup>2+</sup>-exchanged
H-ZSM-5 zeolite in the absence and the presence of molecular oxygen
has been studied with <sup>13</sup>C magic angle spinning (MAS) NMR
spectroscopy. It has been established that the methane activation
on zinc cationic sites under nonoxidative conditions occurs exclusively
by an āalkylā pathway to form the surface zinc-methyl
species. The addition of the molecular oxygen (dioxygen) to methane
adsorbed on the Zn<sup>2+</sup>-exchanged H-ZSM-5 zeolite results
in the surface methoxy and other oxygen-containing species, such as
formate, acetaldehyde, and acetic acid. The formation of the surface
methoxy species occurs by the oxidation with molecular oxygen of zinc-methyl
species primarily formed on the zeolite surface. The Zn<sup>2+</sup>/ZSM-5 zeolite with full substitution of BrĆønsted acid sites
(BAS) by Zn<sup>2+</sup> cations offers zinc-methyl species from methane
at <i>T</i> ā„ 523 K, whereas Zn<sup>2+</sup>/H-ZSM-5
with partial substitution (60%) of BAS produces zinc-methyl at room
temperature. BAS promotes the formation and decomposition (by the
sample evacuation) of zinc-methyl species on Zn<sup>2+</sup>/H-ZSM-5
at room temperature. Zinc-methyl is readily oxidized by the dioxygen
additive to offer methoxy species already at room temperature. Thus,
it has been shown that pure methane forms only zinc-methyl species
upon its interaction with zinc cationic sites of Zn<sup>2+</sup>-exchanged
H-ZSM-5 zeolite, while the surface methoxide could be formed only
by the interaction of zinc-methyl with dioxygen that might be contained
in the reactive methane