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 gold­(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 branch–branch 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 H₂O, CO₂, 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 KHCO₃ 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 β-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 Metal–Organic Frameworks by ¹H Solid-State NMR Spectroscopy

Metal–organic 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. ¹H 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 ¹H SSNMR is severely limited by the strong ¹H–¹H homonuclear dipolar coupling. In this work, we demonstrate that high-resolution ¹H 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 α-Mg₃(HCOO)₆ and α-Mg₃(HCOO)₆ 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 ¹H 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 ¹H–¹H proximity maps obtained from 2D ¹H–¹H double quantum (DQ) experiments in conjunction with theoretical calculations. ¹H MAS spectra of framework hydrogen are very sensitive to the guest molecules present inside the pores and they provide insight into host–guest interaction and dynamics of guest molecule. The ability of achieving very high resolution for ¹H 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/Al₂O₃ catalysts were examined by combining measured rates with theoretical estimates provided by partition function based analyses. Normal H/D isotope effects (<i>r</i>_H/<i>r</i>_D > 1) were observed for endocyclic and exocyclic C–C bond hydrogenolysis. Hydrogenolysis is concluded to occur via stepwise dehydrogenation followed by cleavage of the C–C bond and subsequent hydrogenation of the cleaved entities. The so-called “multiplet” mechanism (i.e., the C–C 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 C–C bond cleavage or C–H­(D) bond reformation may be rate-determining, due to their indistinguishable isotope effects under the studied conditions. C–H­(D) bond dissociation does not control the rate of C–C 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 Zn­(II) from pH 3.5 to 14. 2D homo- and heteronuclear correlation NMR experiments were utilized to extract the ¹H, ¹³C, and ¹⁵N chemical shifts in various histidine–Zn­(II) binding complexes. Several histidine–Zn­(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 Zn­(II) at acidic pH. At pH 7.5, one zinc ion could be hexa-coordinated with two histidine molecules on C′, N_α and deprotonated N_δ1 sites. As the pH increases to 11–14, both of the N_δ1 and N_ε2 sites could be deprotonated as acceptors to be bound to either Zn­(II) or water. All of these findings give a comprehensive set of benchmark values for NMR parameters and structural geometries in variable histidine–Zn­(II) binding complexes over a wide pH range and might provide insights into the structure–property relationship of histidine–metal 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^–1, some of them were characterized by considerably higher energies (>32.8 kJ mol^–1) 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^–1), 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 Metal–Organic 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 metal–organic 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, π 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 “dual-cycle” 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 Å in pore size, the diffusion behaviors of some representative species following “dual-cycle” 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. 2–3 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 “paring” or “side-chain” 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 (≤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 ¹³C 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 Brønsted 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