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In the present work, ultrafast crystallization of AlPO4-5 molecular sieve was observed in a deep eutectic solvent (DES) composed of tetraethylammonium chloride (TEACl) and pentaerythritol (PE). Und...

Metal–organic frameworks (MOFs) are a class of porous materials that have attracted much attention for gas adsorption applications. The ultramicroporous SIFSIX-3-Zn MOF, made up of zinc and pyrazin...

Silicoaluminophosphates (SAPOs) are microporous frameworks with Bronsted acid sites that can be used as acidic catalysts. A firm understanding of SAPO structure, formation, and crystallinity is nec...

Small-pore molecular sieve SAPO-34 is well-known as the most promising and effective catalyst for the conversion of methanol to olefins (MTO). In this paper, we have investigated the formation of SAPO-34 under different dry gel conversion (DGC) conditions using two types of synthesis gels with one containing hydrofluoric acid (HF) and the other not. Particular attention was paid to the Si incorporation and distribution in final SAPO-34 products. The results indicate that under DGC (with HF) conditions SAPO-34 is formed from the transformation of a highly crystalline, layered prephase that is held by covalent bonds. The final SAPO-34 product is a mixture of triclinic and trigonal phases containing six framework Si species. The Si distribution in triclinic and trigonal SAPO-34 is drastically different. The majority of Si species are located in the aluminosilicate domains in trigonal phase. Conversely, under DGC (without HF) conditions, the crystallization of SAPO-34 involves three transformational stages. I...

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 sensitivit...

Metal-organic frameworks (MOFs) are a class of novel nanoporous materials with many potential applications. Structural characterization is important because understanding the relationship between the properties of these industrially relevant materials and their structures allows one to develop new applications and improve current performance. Oxygen is one of the most important elements in many MOFs and exists in various forms. Ideally, 17O solid-state NMR (SSNMR) should be an excellent tool for characterizing various oxygen species. However, the major obstacles that prevent applying 17O SSNMR to MOF characterization are the synthetic effort needed for 17O isotopic enrichment and the associated high cost. In this work, we successfully prepared several prototypical 17O-enriched MOFs, including Zr-UiO-66, MIL-53(Al), CPO-27-Mg (or Mg-MOF-74), and microporous α-Mg3(HCOO)6. Depending on the target MOF, different isotopic enrichment methods were used to effectively incorporate 17O from 17O-enriched H2O. Using these 17O-enriched MOFs, we were able to acquire 17O SSNMR spectra at a magnetic field of 21.1 T. They provide distinct spectral signatures of various key oxygen species commonly seen in representative MOFs. We demonstrate that 17O SSNMR can be used to differentiate chemically and, under favorite circumstances, crystallographically nonequivalent oxygens and to follow the phase transitions. The synthetic approaches for preparation of 17O-enriched sample described in this paper are fairly simple and cost-effective.

The capabilities of silver catalysts with different particle sizes for oxygen activation were systematically studied by O-17 solid-state NMR. The observation of O-17 signal at around 0 ppm for silver catalysts indicates that silver activates O-17(2) molecules and that subsequent formation of active O-17 species leads to the oxygen exchange with the mesoporous SiO2 supports. The initial appearances of the O-17 signals at different temperatures for three silver catalysts show that oxygen activation strongly depends on the size of Ag particle. Their ability of activating oxygen molecules is in the order Ag-3/MCM-41 > Ag-5/SBA-15 > Ag-10/SBA-15. Ag-3/MCM-41 is able to activate oxygen molecules and facilitates the oxygen exchange with the support even at room temperature. Although the NMR signal of active O-17 species on silver catalysts is not observed directly, the approach described in this work can be used to indirectly determine the conditions on which the active oxygen species can be produced during catalytic oxidation reactions. The silver catalysts can also be used for O-17 enrichment of the oxide-based materials under mild conditions.

Layered and open framework zirconium phosphates (ZrPs) have many current and potential applications in the areas of catalysis, sorption, protonic conductors, solar energy storage, crystal engineering, and ion exchange. Characterization of ZrP-based materials is important because understanding the relationship between the properties of these materials and their structures is crucial for developing new uses and for improving their performances in current applications. However, local Zr environments in many ZrPs have not been characterized directly by 91Zr solid-state NMR (SSNMR). This is because 91Zn (I = 5/2) is an unreceptive nucleus with many NMR unfavorable characteristics, leading to low sensitivity. In this work, the local environments of the zirconium centers in several ion-exchanged derivatives of layered α-ZrP (K+-, Li+-, Co(NH3)63+-ZrP) have been probed directly using 91Zr MAS, static quadrupolar echo, and/or quadrupolar Carr–Purcell–Meiboom–Gill NMR. Several layered and three-dimensional framewor...

The conformational behavior of trans-1,2-dichlorocyclohexane (T12D) adsorbed inside zeolites with Faujasite structure (FAU) including sodium Y (Na–Y) and siliceous Y (Si–Y) was investigated by FT-Raman spectroscopy and molecular simulations. The results have clearly shown that the conformational and dynamic properties of T12D strongly depend on the presence of charge-balancing cations as well as Si/Al ratio. Upon loading into Na–Y, the population of the diequatorial (ee) conformer increases compared with pure T12D liquid due to the strong interaction with the extra-framework Na+ ions. Molecular simulations of T12D in Na–Y and Si–Y have also been carried out. The T12D molecule was modeled by quantum and semiempirical quantum chemistry methods and embedded in a zeolite framework that was described by empirical force field. Conformational changes were sampled using quantum mechanics/molecular mechanics molecular dynamics and replica exchange molecular dynamics. Molecular simulations revealed that the ee conf...

Crystallization of a catalytically important molecular sieve SAPO-34 was examined under dry gel conversion conditions including steam-assisted conversion (SAC) and vapor-phase transport (VPT). The progressive changes in the structure of solid gel phases were monitored by powder XRD, SEM, and several solid-state NMR techniques involving 31P, 27Al, 29Si, and 13C. The results suggest that the formation pathways under VPT and SAC appear to be similar: SAPO-34 crystallizes from a semicrystalline precursor with a layered structure held mainly by weak nonbonding interactions. AFM data indicate that the crystal growth mechanism is “birth and spread” and that the fast nucleation on the (001) crystal face likely results from simultaneous transformation of many small domains of precursor to SAPO-34 via cooperative bond breaking and reforming. 29Si NMR results suggest that Si incorporation occurs slowly via amorphous aluminosilicate species. These findings provide new physical insights into the formation of SAPO-34.

The behavior of completely siliceous zeolite ZSM- 5 (Si-ZSM-5) under high pressures up to 16 GPa were investigated by in situ Raman spectroscopy and X-ray diffrac- tion with synchrotron radiation in a diamond anvil cell. Pres- sure-induced amorphization was observed in both as-made and calcined Si-ZSM-5, which transform to a low-density amorphous silica first and then to a high-density amorphous silica. However, transition pressures and reversibility were different for as-made and calcined Si-ZSM-5. It was found that the existence of the template molecules occluded in the zeolite framework is mainly responsible for these differences.

The phase transitions of naphthalene and its derivatives including 2-methylnaphthalene, 2-methyoxynaphthalene, and 2-chloronaphthalene confined in mesoporous silicas of spherical and cylindrical pore geometry were investigated by differential scanning calorimetry (DSC), and a depression of the phase transition temperature was observed. It was found that depression and freeze−melt hysteresis are generally greater in spherical pores. Using powder X-ray diffraction, it was observed that the crystal structure of confined naphthalene is the same as the bulk. Using DSC and Raman spectroscopy, it was shown on both a thermodynamic and molecular level that there is a nonfreezing layer on the pore walls for each confined compound and its thickness depends on the pore diameter.

β-Barium Borate (β-BBO or β-BaB2O4) is an important nonlinear optical material with many practical applications. In this work, we have directly probed the local Ba environment in β-BBO by acquiring static 135Ba and 137Ba solid-state NMR spectra at an ultrahigh magnetic field of 21.1 T. The observed spectra are completely dominated by the second-order quadrupolar interaction. The measured quadrupolar coupling constants are 14.9 and 22.8 MHz for 135Ba and 137Ba, respectively. A computational study of the electric field gradient tensor at Ba2+ was also performed using both plane-wave pseudopotentials (CASTEP) and ab initio (Gaussian 03) methods. Combining the experimental and theoretical results allows one to shed some light on the controversy regarding the true space group of β-BBO. We conclude that the space group R3c represents the true structure of β-BBO rather than does the space group R3. This work demonstrates that a combination of 135/137Ba NMR and theoretical calculations is a useful tool for charac...

Crystallization of AlPO4-5 (AFI), a representative aluminophosphate-based molecular sieve with AFI topology, was examined under dry gel conversion, in particular, vapor phase transport (VPT) condit...

Surface-functionalized periodic mesoporous organosilica (PMO) hollow spheres are successfully synthesized by using a hybrid silica precursor, 1,2-bis(trimethoxysilyl)ethane (BTME), and five precursors with different functional groups (−SH, −NH2, −CN, −C═C, −benzene) as well as surfactants, fluorocarbon and cetyltrimethylammonium bromide, combining a new vesicle and liquid crystal “dual templating” technique. Different disruption effects on the final mesostructure are observed following the order of −SH from 3-mercaptopropyltrimethoxysilane (MPTMS) > −benzene from (trimethoxysilyl)benzene (TMSB) ∼ −C═C from vinyltrimethoxysilane (VTMS) > −NH2 from 3-aminopropyltriethoxysilane (APTES) > −CN from 3-cyanopropyltriethoxysilane (CPTES). The particle size, cavity size, and wall thickness of these hollow spheres can be adjusted by changing the amount of precursors or surfactants applied. In terms of providing better control over surface properties of products and giving more uniform surface coverage of functional...

Layered titanium phosphates (TiPs) have many potentially important applications in ion exchange, catalysis, intercalation, and sorption. Characterization of metal local environments by solid-state 47/49Ti NMR has been difficult due to many unfavorable 47/49Ti NMR properties. In this work, we have directly characterized the local structures around Ti in several representative layered TiPs, including α-, β-, and γ-TiP, by examining the 47/49Ti static NMR spectra of these materials at an ultrahigh magnetic field of 21.1 T. The 47/49Ti chemical shielding and electric field gradient (EFG) tensors have been extracted from spectral analysis. The observed 47/49Ti spectra are mainly determined by the second-order quadrupolar interactions. The quadrupole coupling constants (CQ) are sensitive to the distortion of the TiO6 octahedron in this series of layered TiPs. Quantum mechanical calculations have been performed on several model clusters as well as periodic systems. The results indicate that, in addition to the o...

The adsorption of toluene in partially Li+-exchanged zeolite K-L (LiK-L) was examined by solid-state NMR spectroscopy. The environment of the Li+ ions at different sites in the zeolite before and after adsorption was characterized by 7Li magic-angle spinning (MAS) NMR. 1H → 7Li cross polarization (CP) and 7Li{1H} rotational-echo double-resonance (REDOR) experiments were performed to probe the cation−sorbate interactions. The data were also used for spectral assignment. The 7Li NMR results indicate that most of the Li+ ions occupy the sites A (39%) and B (43%) and that there are fewer Li+ ions at sites C (17%) and D (1%). The four 7Li NMR signals corresponding to the Li+ ions at four sites are not resolved in the spectrum of the dehydrated zeolite, but are well-separated after the adsorption of toluene. The NMR results show that a toluene molecule is directly coordinated to the Li+ ion at site D near the wall of the main channel via formation of a π-complex, resulting in a significant change in 7Li chemica...

Layered zirconium phosphates (ZrPs) and their derivatives potentially have many important applications. In the past, these materials have been mainly characterized by X-ray diffraction and 31P MAS NMR. Their metal centers have not been directly probed by solid-state NMR spectroscopy. In this work, we present solid-state 91Zr NMR spectra acquired at several magnetic fields for several representative layered zirconium phosphates including α-Zr(HPO4)2·H2O, γ-Zr(PO4)(H2PO4)·2H2O, Zr(NH4PO4)2·H2O, and Zr2(NaPO4)4·6H2O. The NMR interaction tensors were extracted from the spectra. The results indicate that the 91Zr spectra are sensitive not only to the relatively small distortion in ZrO6 polyhedron but also to the difference in the geometry of Zr(OP)6 units in these materials. We show that 91Zr quadrupolar coupling constants (CQ) correlate well with several angular distortion parameters reflecting the deviation from a perfect ZrO6 octahedron such as distortion index, shear strain, and mean O−Zr−O angle. The rela...

Crystallization of a representative aluminophosphate microporous material, namely AlPO4-11 with AEL structure, has been investigated by dry-gel conversion (DGC) including steam-assisted conversion (SAC) and vapor-phase transport (VPT). The crystallization under SAC and VPT conditions appears to follow the same pathway: the initial amorphous material is converted to a semicrystalline intermediate, which transforms into AlPO4-11. The semicrystalline intermediate appears to have a three-dimensional structure that bears some similarity to AEL structure. Raman spectrum suggests that it contains the 10-ring channel. XRD patterns indicate that the structure seems to be only ordered in the layer associated with the unit cell ab plane of AlPO4-11. The layers are stacked together via H-bonding and van der Waals interactions. However, the layers are not perfectly aligned as they are in the AEL structure, resulting in disordering along the 10-ring channel direction. Quantitative 31P MAS NMR spectra confirm the amorp...

The adsorption of toluene inside partially Li+-exchanged zeolite Y (LiNa−Y) was examined by solid-state NMR spectroscopy. The environments of Li+ and Na+ ions at different sites in the framework before and after adsorption were characterized by 7Li and 23Na magic-angle-spinning (MAS) NMR. The information on the dynamic behavior of guest molecules inside the supercage of the zeolite was obtained from wide-line 2H NMR spectra. The cation−sorbate interactions were directly probed by heteronuclear dipolar coupling based double-resonance experiments such as 7Li{1H} and 23Na{1H} rotational-echo double-resonance (REDOR) experiments. Molecular modeling was also performed to assist in interpreting NMR results. The 7Li and 23Na MAS results indicate that during ion exchange the incoming Li+ ions fill the SI‘ sites first and then the SII sites, and the remaining Na+ ions stay mostly at SII sites. The MAS and REDOR results show that the toluene molecules are facially coordinated to the cations at SII sites in the supe...

Binary cadmium chalcogenide materials (CdS, CdSe, and CdTe) have been successfully synthesized at room temperature in the mesoporous environments of MCM-41 and MCF (mesocellular foam) by utilizing silylated-chalcogen reagents [E(SiMe3)2, E = S, Se, Te] as an efficient delivery source of E2-. The encapsulated materials are easily prepared by the initial complexation of anhydrous cadmium acetate to ethylenediamine functionalized mesoporous material. The subsequent addition of E(SiMe3)2 leads to the preferential formation of CdE materials within the host. The observed blue shift in absorption maximum is in agreement with the expected quantum confinement of these materials given the nanometer dimensions of the mesoporous architecture. Mild thermal treatment of CdS and CdSe composites demonstrates the ability to control particle growth under specified thermal conditions, ultimately leading to a red shift in absorption maximum upon increasing thermolysis temperature. The utility of silylated-chalcogen reagents ...