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

Microporous Aluminophosphate ULM-6: Synthesis, NMR Assignment, and Its Transformation to AlPO₄‑14 Molecular Sieve

A pure fluorinated aluminophosphate [Al₈P₈O₃₂F₄·(C₃H₁₂N₂)₂(H₂O)₂] (ULM-6) has been synthesized via an aminothermal strategy, in which triisopropanolamine (TIPA) is used as the solvent together with the addition of propyleneurea and HF. The ¹³C NMR spectrum demonstrates that 1,3-diaminopropane, the <i>in situ</i> decomposer of propyleneurea, is the real structure-directing agent (SDA) for ULM-6 crystals. The local Al, P, and F environments of the dehydrated ULM-6 are investigated by 1D and 2D solid-state NMR spectroscopy. The spatial proximities are extracted from ¹⁹F­{²⁷Al}, ¹⁹F­{³¹P}, ²⁷Al­{¹⁹F}, and ³¹P­{¹⁹F} rotational-echo double resonance (REDOR) NMR experiments as well as ¹⁹F → ³¹P heteronuclear correlation (HETCOR) NMR and {³¹P}²⁷Al HMQC NMR experiments, allowing a full assignment of all the ¹⁹F, ²⁷Al, and ³¹P resonances to the corresponding crystallographic sites. Moreover, it is found that the structure of ULM-6 is closely related to that of AlPO₄-14. A combination of high-temperature powder XRD, thermal analysis, and ¹⁹F NMR reveals that the removal of fluorine atoms at higher temperature is crucial to the phase transformation of ULM-6 to AlPO₄-14. The calcined product shows high CO₂/CH₄ and CO₂/N₂ selectivity with ratios of 15.5 and 29.1 (101 kPa, 25 °C), respectively

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

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

Supplementary_Information - Ternary memory property of novel polyimide with backbone carbazole moiety and pendant triphenylamine moiety

<p>Supplementary_Information for Ternary memory property of novel polyimide with backbone carbazole moiety and pendant triphenylamine moiety by Yanhua Yang, Pan Jin, Jinzhang Liu, Shijin Ding, Lin Chen, Yueying Chu, and Yingzhong Shen in High Performance Polymers</p

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

Inspecting the Structure and Formation of Molecular Sieve SAPO-34 via ¹⁷O Solid-State NMR Spectroscopy

Silicoaluminophosphates (SAPOs) are microporous frameworks with Brønsted acid sites that can be used as acidic catalysts. A firm understanding of SAPO structure, formation, and crystallinity is necessary for understanding and expanding SAPO applications in heterogeneous catalysis. Solid-state ¹⁷O NMR (SSNMR) spectroscopy is an ideal tool to probe structure and formation of SAPO-based materials; the ¹⁷O quadrupolar and chemical shift interactions are exquisitely sensitive to local electronic and magnetic environments. In this work, a pure trigonal SAPO-34 molecular sieve synthesized via the dry-gel conversion (DGC) method was investigated using a combination of ¹⁷O magic-angle spinning, ¹⁷O triple-quantum magic-angle spinning, ¹⁷O­[²⁷Al] transfer of population in double-resonance, and ¹⁷O­[³¹P] rotational-echo double-resonance SSNMR spectroscopy, complemented by powder X-ray diffraction along with ²⁷Al, ²⁹Si, and ³¹P multinuclear SSNMR experiments. The four observed ¹⁷O resonances were simulated to extract NMR parameters, with each resonance assigned to individual oxygen local environments and connectivities in SAPO-34. The incorporation of oxygen from ¹⁷O-labeled water (i.e., H₂¹⁷O) during the DGC formation of pure trigonal SAPO-34 has also been investigated at various time intervals and stages of crystallization using ¹⁷O SSNMR. There is direct involvement of ¹⁷O-enriched water vapor during the DGC crystallization process of trigonal SAPO-34. The initial dry gel is amorphous, transforming to a crystalline layered AlPO₄ phase during the first hour of heating and then progressing to a semicrystalline phase after 4 h of heating; formation of the crystalline trigonal SAPO-34 product was found to be complete after 2 days

Spies Within Metal-Organic Frameworks: Investigating Metal Centers Using Solid-State NMR

Structural characterization of metal–organic frameworks (MOFs) is crucial, since an understanding of the relationship between the macroscopic properties of these industrially relevant materials and their molecular-level structures allows for the development of new applications and improvements in current performance. In many MOFs, the incorporated metal centers dictate the short- and long-range structure and porosity of the material. Here we demonstrate that solid-state NMR (SSNMR) spectroscopy targeting NMR-active metal centers at natural abundance, in concert with ab initio density functional theory (DFT) calculations and X-ray diffraction (XRD), is a powerful tool for elucidating the molecular-level structure of MOFs. ⁹¹Zr SSNMR experiments on MIL-140A are paired with DFT calculations and geometry optimizations in order to detect inaccuracies in the reported powder XRD crystal structure. ¹¹⁵In and ¹³⁹La SSNMR experiments on sets of related MOFs at two different magnetic fields illustrate the sensitivity of the ¹¹⁵In/¹³⁹La electric field gradient tensors to subtle differences in coordination, bond length distribution, and ligand geometry about the metal center. ^47/49Ti SSNMR experiments reflect the presence or absence of guest solvent in MIL-125­(Ti), and when combined with DFT calculations, these SSNMR experiments permit the study of local hydroxyl group configurations within the MOF channels. ⁶⁷Zn SSNMR experiments and DFT calculations are also used to explore the geometry near Zn within a set of four MOFs as well as local disordering caused by distributions of different linkers around the metal. SSNMR spectroscopy of metal centers offers an impressive addition to the arsenal of techniques for MOF characterization and is particularly useful in cases where XRD information may be ambiguous, incomplete, or unavailable

Insights of the Crystallization Process of Molecular Sieve AlPO₄‑5 Prepared by Solvent-Free Synthesis

Crystallization of AlPO₄-5 with AFI structure under solvent-free conditions has been investigated. Attention was mainly focused on the characterization of the intermediate phases formed at the early stages during the crystallization. The development in the long-range ordering of the solid phases as a function of crystallization time was monitored by XRD, SEM, IR, UV-Raman, and MAS NMR techniques. Particularly, the UV-Raman spectroscopy was employed to obtain the information on the formation process of the framework. <i>J</i>-HMQC ²⁷Al/³¹P double-resonance NMR experiments were used to identify the P–O–Al bonded species in the intermediate phases. For the first time the P–O–Al bonded species in the intermediate phases can be correctly described through using this advanced NMR technique. The crystallization under solvent-free conditions appears to follow the pathway: The initial amorphous raw material is converted to an intermediate phase which has four-/six-membered ring species, then gradually transformed into crystalline AlPO₄-5. This observation is not consistent with the common idea that the intermediate phase is the semicrystalline intermediates with a three-dimensional structure

Highly Mesoporous Single-Crystalline Zeolite Beta Synthesized Using a Nonsurfactant Cationic Polymer as a Dual-Function Template

Mesoporous zeolites are useful solid catalysts for conversion of bulky molecules because they offer fast mass transfer along with size and shape selectivity. We report here the successful synthesis of mesoporous aluminosilicate zeolite Beta from a commercial cationic polymer that acts as a dual-function template to generate zeolitic micropores and mesopores simultaneously. This is the first demonstration of a single nonsurfactant polymer acting as such a template. Using high-resolution electron microscopy and tomography, we discovered that the resulting material (Beta-MS) has abundant and highly interconnected mesopores. More importantly, we demonstrated using a three-dimensional electron diffraction technique that each Beta-MS particle is a single crystal, whereas most previously reported mesoporous zeolites are comprised of nanosized zeolitic grains with random orientations. The use of nonsurfactant templates is essential to gaining single-crystalline mesoporous zeolites. The single-crystalline nature endows Beta-MS with better hydrothermal stability compared with surfactant-derived mesoporous zeolite Beta. Beta-MS also exhibited remarkably higher catalytic activity than did conventional zeolite Beta in acid-catalyzed reactions involving large molecules