13 research outputs found
In Situ Monitoring of Heterogeneous Catalytic Hydrogenation via 129Xe NMR Spectroscopy and Proton MRI
Branching-chain propagation of parahydrogen-derived nuclear spin order on a catalyst surface
When a parahydrogen molecule dissociates on a surface of a heterogeneous catalyst (e.g., of a metal nanoparticle), the correlation of the nuclear spins initially inherited by the two surface H atoms may be shared with other surface hydrogens as they diffuse and combine with random H atoms to produce H2 molecules which subsequently dissociate. This branching-chain-type propagation of nuclear spin order leads to its gradual dilution but at the same time is accompanied by an increase in the number of H atoms that share nuclear spin order. These conclusions, confirmed by the spin density matrix calculations, may be relevant in the context of parahydrogen-induced polarization (PHIP) in heterogeneous hydrogenations catalyzed by supported metal catalysts, observation of which apparently contradicts the accepted non-pairwise mechanism of hydrogen addition to an unsaturated substrate over such catalysts. The potential consequences of the reported findings are discussed in the context of PHIP effects and beyond
Pairwise hydrogen addition in the selective semihydrogenation of alkynes on silica-supported Cu catalysts
Mechanistic insight into the semihydrogenation of 1-butyne and 2-butyne on Cu nanoparticles supported on partially dehydroxylated silica (Cu/SiO2-700) was obtained using parahydrogen. Hydrogenation of 1-butyne over Cu/SiO2-700 yielded 1-butene with ā„97% selectivity. The surface modification of this catalyst with tricyclohexylphosphine (PCy3) increased the selectivity to 1-butene up to nearly 100%, although at the expense of reduced catalytic activity. Similar trends were observed in the hydrogenation of 2-butyne, where Cu/SiO2-700 provided a selectivity to 2-butene in the range of 72ā100% depending on the reaction conditions, while the catalyst modified with PCy3 again demonstrated nearly 100% selectivity. Parahydrogen-induced polarization effects observed in hydrogenation reactions catalyzed by copper-based catalysts demonstrate the viability of pairwise hydrogen addition over these catalysts. Contribution of pairwise hydrogen addition to 1-butyne was estimated to be at least 0.2ā0.6% for unmodified Cu/SiO2-700 and ā„2.7% for Cu/SiO2-700 modified with PCy3, highlighting the effect of surface modification with the tricyclohexylphosphine ligand.ISSN:2041-6520ISSN:2041-653
Chemical Reaction Monitoring Using Zero-Field Nuclear Magnetic Resonance Enables Study of Heterogeneous Samples in Metal Containers
We
demonstrate that heterogeneous/biphasic chemical reactions can be monitored with
high spectroscopic resolution using zero-field nuclear magnetic resonance. This
is possible because magnetic susceptibility broadening is insignificant at
ultralow magnetic fields. We show the two-step hydrogenation of dimethyl
acetylenedicarboxylate with para-enriched hydrogen gas in conventional
glass NMR tubes, as well as in a titanium tube. The low frequency zero-field
NMR signals ensure that there is no significant signal attenuation due to
shielding by the electrically conductive sample container. This method paves
the way for in situ monitoring of reactions in complex heterogeneous
multiphase systems and in reactors made from conductive materials without
magnetic susceptibility induced line broadening.</div
Chemical Reaction Monitoring using ZeroāField Nuclear Magnetic Resonance Enables Study of Heterogeneous Samples in Metal Containers
Manipulating Stereoselectivity of Parahydrogen Addition to Acetylene to Unravel Interconversion of Ethylene Nuclear Spin Isomers
Symmetric molecules exist as distinct nuclear spin isomers (NSIMs). A deeper understanding of their properties, including interconversion, requires efficient techniques for NSIMs enrichment. Selective hydrogenation of acetylene with parahydrogen (p-H2) was used to achieve the enrichment of ethylene NSIMs and to study their equilibration processes. The effect of stereoselectivity of H2 addition to acetylene on the imbalance of ethylene NSIMs was experimentally demonstrated by using different heterogeneous catalysts (an immobilized Ir complex and two supported Pd catalysts). The interconversion of NSIMs with time during ethylene storage was studied with NMR spectroscopy by reacting ethylene with bromine water which renders the p-H2-derived protons in the produced 2-bromoethan(2H)ol (BrEtOD) magnetically inequivalent, thereby revealing the non-equilibrium nuclear spin order of ethylene. A thorough analysis of the shape and transformation of the 1H NMR spectra of hyperpolarized BrEtOD allowed us to reveal the initial distribution of produced ethylene NSIMs and their equilibration processes. Comparison of the results obtained with different catalysts was key to properly attributing the derived characteristic time constants to different NSIMs interconversion processes: ~ 3-6 s for interconversion between NSIMs with the same inversion symmetry (i.e., within g or u manifolds) and ~ 1700-2200 s between NSIMs with different inversion symmetries
Bimetallic PdāAu/Highly Oriented Pyrolytic Graphite Catalysts: from Composition to Pairwise Parahydrogen Addition Selectivity
The
model Pd and Au mono- and bi-metallic (PdāAu) catalysts
were prepared using vapor deposition of metals (Au and/or Pd) under
ultrahigh vacuum conditions on the defective highly oriented pyrolytic
graphite (HOPG) surface. The model catalysts were investigated using
the X-ray photoelectron spectroscopy and scanning tunneling microscopy
at each stage of the preparation procedure. For the preparation of
bimetallic catalysts, different procedures were used to get different
structures of PdAu particles (Au<sub>shell</sub>āPd<sub>core</sub> or alloyed). All prepared catalysts showed rather narrow particles
size distribution with an average particles size in the range of 4ā7
nm. Parahydrogen-enhanced nuclear magnetic resonance spectroscopy
was used as a tool for the investigation of PdāAu/HOPG, Pd/HOPG,
and Au/HOPG model catalysts in propyne hydrogenation. In contrast
to Au sample, Pd, PdAu<sub>alloy</sub>, and Au<sub>shell</sub>āPd<sub>core</sub> samples were shown to have catalytic activity in propyne
conversion, and pairwise hydrogen addition routes were observed. Moreover,
bimetallic samples demonstrated the 2- to 5-fold higher activity in
pairwise hydrogen addition in comparison to the monometallic Pd sample.
It was shown that the structures of bimetallic PdāAu particles
supported on HOPG strongly affected their activities and/or selectivities
in propyne hydrogenation reaction: the catalyst with the Au<sub>shell</sub>āPd<sub>core</sub> structure demonstrated higher pairwise
selectivity than that with the PdAu<sub>alloy</sub> structure. Thus,
the reported approach can be used as an effective tool for the synergistic
effects investigations in hydrogenation reactions over model bimetallic
PdāAu catalysts, where the active component is supported on
a planar support
Deciphering the Nature of Ru Sites in Reductively Exsolved Oxides with Electronic and Geometric MetalāSupport Interactions
The reductive exsolution of metallic Ru from fluorite-type solid solutions Ln2Ru0.2Ce1.8O7 (Ln = Sm, Nd, La) leads to materials with metalāsupport interactions that influence the electronic state and the catalytic activity of Ru. In situ X-ray absorption spectroscopy at the Ru K-edge identified that with increasing temperature, the exsolution of Ru from Sm2Ru0.2Ce1.8O7 in a H2 atmosphere proceeds via an intermediate RuĪ“+ state, that is, Ru4+āRuĪ“+āRu0. X-ray photoelectron spectroscopy (XPS) established that, in parallel (H2 atmosphere at ca. 500 Ā°C), also Ce4+ ions reduce to Ce3+, which is accompanied by an electron transfer from the reduced host oxide to the exsolved Ru0 clusters, creating RuĪ“ā states. Low-temperature diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) using CO as a probe molecule reveals a red shift of the CO adsorption bands by ca. 18 cmā1 when increasing the temperature during the H2 treatment from 300 to 500 Ā°C, consistent with an increased Ļ-backdonation from more electron-rich Ru species to CO. However, at a lower reduction temperature of ca. 100 Ā°C, a blue-shifted CO band is observed that is explained by a Lewis-acidic RuĪ“+āCO adduct. Nuclear magnetic resonance (NMR) signal enhancement in parahydrogen-induced polarization experiments was used as a structure-sensitive probe and revealed a decreasing propene hydrogenation rate with increasing exsolution temperature, accompanied by a notable enhancement of propane hyperpolarization (ca. 3-fold higher at 500 Ā°C than at 300 Ā°C). These data suggest that the exsolved, subnanometer-sized Ru species are more active in propene hydrogenation but less selective for the pairwise addition of p-H2 to propene than RuĪ“ā sites engaged in a strong metalāsupport interaction. Ā© 2020 American Chemical Society.ISSN:1932-7455ISSN:1932-744
Hydrogenation of Unsaturated Six-Membered Cyclic Hydrocarbons Studied by the Parahydrogen-Induced Polarization Technique
Parahydrogen-induced polarization
(PHIP) is an efficient technique
for mechanistic investigations of homogeneous and heterogeneous catalytic
hydrogenations. Herein, heterogeneous gas phase hydrogenation of six-membered
cyclic hydrocarbons (benzene, toluene, cyclohexene, 1,3-cyclohexadiene
and 1,4-cyclohexadiene) over Rh/TiO<sub>2</sub>, Pd/TiO<sub>2</sub>, and Pt/TiO<sub>2</sub> catalysts was studied using PHIP. As expected,
cyclohexene hydrogenation led to the formation of cyclohexane which
because of its symmetry should not exhibit any PHIP effects. However,
the presence of <sup>13</sup>C nuclei at natural abundance (1.1%)
breaks molecular symmetry, resulting in the observation of <sup>13</sup>C satellite signals exhibiting PHIP effects in the <sup>1</sup>H
NMR spectra. In experiments with cyclohexene, the reactantās
NMR signals were also polarized, demonstrating the possibility of
cyclohexene dehydrogenation to 1,3-cyclohexadiene and subsequent hydrogenation
to cyclohexene. In the hydrogenation of 1,3-cyclohexadiene and 1,4-cyclohexadiene,
all NMR signals of cyclohexene exhibited PHIP effects, implying migration
of Cī»C bonds in 1,4-cyclohexadiene and cyclohexene. At the
same time, upon hydrogenation of benzene and toluene the reaction
products were those with saturated cycles exclusively (cyclohexane
and methylcyclohexane, respectively), and their NMR signals were not
polarized. The absence of PHIP effects for arene hydrogenation can
be explained by a difference in the reaction mechanism compared to
cyclohexane and cyclohexadienes hydrogenations, along with the larger
extent to which hydrogen atoms undergo migration on the catalyst surface
facilitated by lower catalyst coverage with an adsorbed substrate
in case of arenes