Bridging Hydroxy Groups Research Articles

Crown ether-like octanuclear molybdenum(V) clusters for cation binding and gas adsorption.

Octanuclear polyoxomolybdenum-based porous materials, Na8[Mo8O8(μ2-O)8(μ2-OH)8(3-apz)4]2·26H2O (1, 3-Hapz = 3-aminopyrazole), K8[Mo8O8(μ2-O)8(μ2-OH)8(3-apz)4]2·7H2O (2) and (NH4)4[Mo8O8(μ2-O)8(μ2-OH)4(3-apz)8]·20.5H2O (3), have been successfully synthesized by a hydrothermal method and fully characterized. X-ray structural analyses show that microporous materials 1-3 contain round pores formed by octanuclear molybdenum-oxygen groups connected sequentially with pore sizes of 4.0, 4.0, and 4.8 Å, respectively. Both 1 and 2 are composed of two {Mo8} rings, which are connected by strong intramolecular hydrogen bonds between bridging hydroxy groups and oxygen atoms to form dimeric structures. The central pores in 1 and 2 are occupied by Na+ and K+, respectively, while they are empty in 3. This reflects the structural expansion and contraction effects induced by different cations. Through intermolecular stacking, 1-3 also exhibit channels with sizes of 14.0 × 6.4, 4.6 × 2.6, and 5.4 × 5.4 Å, respectively, which were used for the studies of gas adsorption. The results show that 1-3 can selectively adsorb CO2 and O2, including the empty hole in 3, while they show little or no affinity for gases H2, N2, and CH4. Moreover, an additional polyoxomolybdenum-based species (Mo8O26)n·4n(3-H2apz) (4) has been obtained with protonated 3-aminopyrazole in the absence of a reducing agent, which can serve as an intermediate for the polyoxomolybdenum-based porous products.

Two Aluminophosphate Molecular Sieves Built from Pairs of Enantiomeric Structural Building Units

AbstractHerein we report the synthesis and structures of two new small‐pore aluminophosphate molecular sieves PST‐13 and PST‐14 with mutually connected 8‐ring channels. The structure of PST‐13, synthesized using diethylamine as an organic structure‐directing agent, contains penta‐coordinated framework Al atoms bridged by hydroxy groups and thus edge‐sharing 3‐ and 5‐rings. Upon calcination, PST‐13 undergoes a transformation to PST‐14 with loss of bridging hydroxy groups and occluded organic species. The structures of both materials consist “nonjointly” of pairs of previously undiscovered 1,5‐ and 1,6‐open double 4‐rings (d4rs) which are mirror images of each other. We also present a series of novel chemically feasible hypothetical structures built from 1‐open d4r (sti) or 1,3‐open d4r (nsc) units, as well as from these two enantiomeric structural building units.

Two Aluminophosphate Molecular Sieves Built from Pairs of Enantiomeric Structural Building Units

Herein we report the synthesis and structures of two new small-pore aluminophosphate molecular sieves PST-13 and PST-14 with mutually connected 8-ring channels. The structure of PST-13, synthesized using diethylamine as an organic structure-directing agent, contains penta-coordinated framework Al atoms bridged by hydroxy groups and thus edge-sharing 3- and 5-rings. Upon calcination, PST-13 undergoes a transformation to PST-14 with loss of bridging hydroxy groups and occluded organic species. The structures of both materials consist "nonjointly" of pairs of previously undiscovered 1,5- and 1,6-open double 4-rings (d4rs) which are mirror images of each other. We also present a series of novel chemically feasible hypothetical structures built from 1-open d4r (sti) or 1,3-open d4r (nsc) units, as well as from these two enantiomeric structural building units.

An Aluminophosphate Molecular Sieve with 36 Crystallographically Distinct Tetrahedral Sites

AbstractThe structure of the new medium‐pore aluminophosphate molecular sieve PST‐6 is determined by the combined use of rotation electron diffraction tomography, synchrotron X‐ray powder diffraction, and computer modeling. PST‐6 was prepared by calcination of another new aluminophosphate material with an unknown structure synthesized using diethylamine as a structure‐directing agent, which is thought to contain bridging hydroxy groups. PST‐6 has 36 crystallographically distinct tetrahedral sites in the asymmetric unit and is thus crystallographically the most complex zeolitic structure ever solved.

An Aluminophosphate Molecular Sieve with 36 Crystallographically Distinct Tetrahedral Sites

The structure of the new medium-pore aluminophosphate molecular sieve PST-6 is determined by the combined use of rotation electron diffraction tomography, synchrotron X-ray powder diffraction, and computer modeling. PST-6 was prepared by calcination of another new aluminophosphate material with an unknown structure synthesized using diethylamine as a structure-directing agent, which is thought to contain bridging hydroxy groups. PST-6 has 36 crystallographically distinct tetrahedral sites in the asymmetric unit and is thus crystallographically the most complex zeolitic structure ever solved.

Characterization of Thermally Stable Brønsted Acid Sites on Alumina‐Supported Niobium Oxide after Calcination at High Temperatures

Thermally stable Brønsted acid sites were generated on alumina-supported niobium oxide (Nb2O5/Al2O3) by calcination at high temperatures, such as 1123 K. The results of structural characterization by using Fourier-transform infrared (FTIR) spectroscopy, TEM, scanning transmission electron microscopy (STEM), and energy-dispersive X-ray (EDX) analysis indicated that the Nb2O5 monolayer domains were highly dispersed over alumina at low Nb2O5 loadings, such as 5 wt%, and no Brønsted acid sites were presents. The coverage of Nb2O5 monolayer domains over Al2O3 increased with increasing Nb2O5 loading and almost-full coverage was obtained at a loading of 16 wt%. A sharp increase in the number of hydroxy groups, which acted as Brønsted acid sites, was observed at this loading level. The relationship between the acidic properties and the structure of the material suggested that the bridging hydroxy groups (Nb-O(H)-Nb), which were formed at the boundaries between the domains of the Nb2O5 monolayer, acted as thermally stable Brønsted acid sites.

ChemInform Abstract: Synthesis of SAPO-35 in Non-Aqueous Gels.

The molecular sieve SAPO-35 has been synthesized in a non-aqueous medium for the first time using a new template, hexamethyleneimine. It has been characterized by XRD, SEM, TG–DTA, MAS NMR, FTIR and adsorption techniques. The study indicates that Si replaces mainly the P atoms in the AlPO 4 -35 framework giving rise to three different Bronsted acid bridging hydroxy groups, with IR absorptions at 3580, 3604 and 3631 cm −1 . The three bands are attributed to Si incorporation in different tetrahedral positions in double six-membered rings and single six-membered rings.

Bulk and surface properties of commercial kaolins

Four different commercial kaolins have been characterized by specific surface measurement, X-ray diffraction, transmission electron microscopy, SEM–EDX analysis, TGA measurements, DR–UV–Vis–NIR spectroscopy, transmission IR spectroscopy of the samples as such and after acetonitrile adsorption. A correlation between surface acidity and structural and chemical properties was attempted. Kaolinite surface was almost neutral, with a very small amount of medium acidic centers. These centers gave rise to a weak protonic acidity arising from bridging hydroxy-groups on exposed basal planes (exposed gibbsite-like layers), and silanol groups likely formed on non-basal planes, as well as medium strength Lewis acid sites likely exposed on non-basal planes. Lewis and Brønsted sites may also arise from mineral impurities (e.g. anatase, amorphous silica–alumina).

Zinc–aluminum hydrotalcites as precursors of basic catalysts: Preparation, characterization and study of the activation of methanol

Monophasic ZnAl hydrotalcite-like layered hydroxycarbonate has been prepared by the urea precipitation method. This material has been characterized by Rietveld refinement of the XRPD pattern, DTA-TG and skeletal IR. ZnAl mixed oxides have been prepared by calcinations of this precursor. According to XRPD, after calcinations at 450 °C this material is constituted by poorly crystallized ZnO phase. IR spectroscopy shows the presence of additional spinel-like domains. The surface of this catalyst is dominated by poorly acidic Zn 2+ cations and terminal and bridging hydroxy groups bonded to Zn ions. This catalyst adsorbs methanol dissociatively giving rise to two types of methoxy groups. Type A, likely bridging on Al and Zn ions, is more stable to desorption. Type B, likely terminally bonded to Zn 2+ cations, is more labile with respect to decomposition into CO and H 2. Type B methoxy groups appear to be very ionic and are, likely, those involved in transesterification and polyethoxylation reactions. ZnO and ZnAl 2O 4 particles progressively crystallize and sinter by increasing calcination temperature.

Di-μ-hydroxido-bis[tris(1,1,1,5,5,5-hexafluoroacetylacetonato-κ2O,O′)hafnium(IV)] acetone solvate

The binuclear title compound, [Hf2(C5HF6O2)6(OH)2]·C3H6O, contains an HfIV atom which is eight coordinated and surrounded by three chelating β-diketonato 1,1,1,5,5,5-hexa­fluoro­acetyl­acetonate (hfaa) ligands and two bridging OH groups situated on a twofold rotation axis. The HfO8 coordination polyhedron shows a slightly distorted Archimedean square anti-prismatic coordination with average Hf—O, C—O, C—CMe distances of 2.19 (2), 1.26 (2) and 1.49 (2) Å, respectively, and an O—Hf—O bite angle of 75.3 (5)°. Weak O—H⋯O hydrogen bonding inter­actions are observed between one of the bridging hydr­oxy groups and the disordered solvent mol­ecule.

Catena-Poly[[(2,9-dimethyl-1,10-phenanthroline-κ2 N,N′)lead(II)]-di-μ-2-hydroxybenzoato-κ3 O 1,O 1′:O 2;κ3 O 2:O 1,O 1′

In the title polymeric comound, [Pb(C7H5O3)2(C14H12N2)] n , the PbII atom is located on a twofold rotation axis and is coordinated by two N atoms from one 2,9-dimethyl-1,10-phenanthroline (dmphen) ligand and six O atoms from four 2-hydroxybenzoate anions. The compound forms a zigzag polymeric chain along the c axis through bridging hydroxy groups of two 2-hydroxybenzoate ligands. The crystal packing is stabilized by the intramolecular hydrogen bonding and π–π interactions between dmphen rings of neighboring molecules, with a distance between the ring planes of 3.385 (3) Å.

Orientation of Alkyl-Substituted Aromatic Molecules during Sorption in the Pores of H/ZSM-5 Zeolites

Two energetic contributions of the sorption of benzene, toluene, and p-xylene on the MFI zeolites (ZSM-5 and Silicalite-1) process are identified. One is the nonspecific interaction with the pore walls, and the other is the specific directed interaction with the Brønsted acidic SiOHAl groups. Although the energetic contribution of the nonspecific interaction is about 10 times larger than the directed interaction, this minor contribution is sufficient to direct the aromatic molecules at low coverage to sorption sites at intersections containing SiOHAl groups, and only at higher loadings, intersections without bridging hydroxy groups are occupied. The sorption of benzene on the SiOHAl groups is described by a dual-site Langmuir isotherm related to two orientations of the aromatic molecule, differing in the decrease of entropy but with the same heat of adsorption. The two sorption structures are manifested by the appearance of two perturbed hydroxy groups in the IR spectra of the zeolite in contact with the aromatic molecule. At low coverage for toluene, two sorption structures with different steric constraints are differentiated, while at higher coverage only the less constrained structure dominates. In contrast, for p-xylene, the Langmuir model was inadequate to describe the sorption isotherm on the SiOHAl groups, which reflects the strongly confined sorption structure resulting from the two alkyl groups. For all three molecules, the initial preferential adsorption at the intersections causes the molecules to be spatially well-separated, randomly oriented with respect to the SiOHAl groups. With increasing coverage, the molecules arrange into the geometrically less constrained sorption structures.

Synthesis and characterization of diorganotin(IV) derivatives from rhodanine (HRd): crystal structures of [(PhCH2)2Sn(Rd)(μ-OH)]2 and [n-Bu2Sn(Rd)(μ-OH)

The diorganotin(IV) complexes, [R2Sn(Rd)(μ-OH)]2 (R = Me (1), PhCH2 (2), n-Bu (3), Ph (4); HRd = rhodanine), have been synthesized and characterized by IR and multinuclear (1H, 13C, 119Sn) NMR spectroscopy. The structures of complexes 2 and 3 have been determined by single-crystal X-ray diffraction. Both crystal structures of 2 and 3 show the presence of asymmetrically bridging hydroxy groups leading to an Sn2O2 unit. Each atom in complex 1 is also coordinated by an N atom of ligand and two C atoms of the alkyl groups, so the Sn environment is based on a trigonal bipyramid. While in complex 2, a weak intermolecular Sn–O interaction has also been found between the two adjacent molecules, so the geometry of the Sn atom can be best described as six-coordinate octahedral. The salient feature of the supramolecular structure of complex 3 is that of a 1D polymer, in which the discrete molecules are connected through weak intermolecular Sn ··· O interactions.

Energetic and entropic contributions controlling the sorption of benzene in zeolites

The energetic and entropic contributions controlling the sorption of benzene on acidic (H/ZSM-5) and non-acidic (Silicalite-1) MFI type materials were studied using gravimetry, calorimetry and in situ IR spectroscopy to follow the qualitative and quantitative interactions of benzene with the pores and the functional SiOH and SiOHAl groups. The model derived to describe the sorption isotherms indicates the presence of sterically constrained sorption structures for benzene in MFI type materials. The interaction of benzene with the pore walls controls the sorption energetically, while the localized interaction with the bridging hydroxy groups contributes only to a minor degree. If benzene is located close to SiOHAl groups perturbed hydroxy groups are formed. Their wavenumber reflects the local sorption geometry of benzene as well as the acid strength of the hydroxy group and the base strength of benzene. Two perturbed hydroxy groups were observed for benzene adsorbed, which are assigned to two orientations of the molecules inside the pores, i.e., with the ring parallel to the pore wall and with the ring being oriented towards the bridging hydroxy groups. At higher coverage benzene adsorbs at SiOHAl groups additionally in an unconstrained environment, most probably at the pore openings.

A study of the localization and accessibility of Brønsted and Lewis acid sites of H-mordenite through the FT-IR spectroscopy of adsorbed branched nitriles

Adsorption of propionitrile (PrN), isobutironitrile (IBN) and pivalonitrile (PN) with H-MOR (Si/Al=10) has been investigated. The accessibility of IBN and, more, of PN are limited, unlike PrN. Terminal silanols ( 3745 cm −1 ), weakly acidic, are located, as Lewis sites, at the external crystal surface. Bridging hydroxy groups absorbing at 3588 cm −1 are inside the side pockets, those absorbing at 3605 cm −1 are in the main channels and those absorbing at 3609 cm −1 are at the intersection between side pockets and main channels. OH’s due to extra framework materials ( 3650 cm −1 ) are in the side pockets.

An FTIR study of the accessibility of the protonic sites of H-mordenites

The interaction of different probes with two H-MOR samples has been studied by IR. In the case of the sample with Si/Al = 10 acetonitrile perturbs all the hydroxy groups while 2,2-dimethylpropionitrile (pivalonitrile) perturbs only very few. Pyridine also perturbs all the hydroxy groups but only some of them protonate the pyridine, the others only H-bond to it. n-Hexane and 2,2-dimethylbutane give the same result, perturbing only some of the bridging hydroxy groups. The results are interpreted by assuming that no OHs are located in the 8-ring channels, and that the hydrocarbons cannot interact (due to the steric hindrance of the methyl group) with the OHs located in the side pockets. On the contrary, the flat molecule pyridine can enter slightly into the side pockets and H-bond with the OHs there. Pivalonitrile interacts only with the OHs which are well exposed in the main channels. It is concluded that the active sites for alkane isomerization are likely exclusively those that are well exposed in the main channels of H-MOR and that Al substitution in the T3 sites probably does not occur. The sample with Si/Al 45, taken as an example of a dealuminated sample, presents many less bridging OHs which are entirely available for interaction with even pivalonitrile.

Diffuse-Reflectance IR Spectra of Methane Adsorbed on NaZSM-5 and HZSM-5 Zeolites

Diffuse-reflectance IR spectra of methane adsorbed on high-silica NaZSM-5 and HZSM-5 zeolites point to a stronger adsorption of methane on sodium cations than on protons. For the asymmetric stretching vibration ν3, this form of adsorption is characterized by a doublet with band maxima at 2980 and 3010 cm–1. For the fully symmetric stretching vibration ν1, it is characterized by a singlet with a maximum at 2880 cm–1. Methane is also adsorbed on NaZSM-5 in a weaker form, which is characterized by absorption bands with maxima at 3002 (ν3) and 2887 (ν1) cm–1. The weaker form of methane adsorption on acidic bridging hydroxy groups of HZSM-5 is characterized by absorption bands at 3001 and 2887 cm–1 (ν3 and ν1, respectively). A difference between this form of adsorption and weak adsorption on sodium-exchanged zeolite reveals itself in the somewhat higher intensity of the band at 2887 cm–1. For methane adsorbed on NaZSM-5, the frequencies of deformational vibrations and a spectrum in the near IR region are obtained for the first time. It was found that the perturbance of adsorbed methane molecules is seen in the spectrum as in the low-frequency shifts of most of the bands that appear due to composite vibrations and overtones and as new adsorption bands that were not observed for gaseous methane.

ITQ-18 a new delaminated stable zeolite

A new delaminated zeolitic material ITQ-18 has been obtained by expanding and exfoliating a laminar precursor of NU-6 (2); while NU-6 (2) has an N2 surface area of 35 m2 g−1 and contains bridging hydroxy groups which are in accessible to pyridine, the ITQ-18 sample prepared has an a external surface area of 588 m2 g−1 with all bridging hydroxy groups being acidic and accessible to pyridine.

NaV3 (OH)6 (SO4 )2 : A Kagomé-Type Vanadium(III) Compound with Strong Intralayer Ferromagnetic Interactions.

A dominant ferromagnetic exchange interaction propagates about the magnetic sites of the Kagomé lattice of the title compound through the bridging hydroxy groups (see section of the structure). This is at variance with the antiferromagnetic exchange observed for jarosite and its derivatives. The ferromagnetism probably arises from the d2 electron count of the VIII centers.

IR microspectroscopic investigation of the acid sites in metal substituted AlPO4-5 molecular sieves Part 1.Sorption of benzene and strong bases

The functional groups of metal substituted aluminophosphate (MeAPO-5) materials and their interactions with probe molecules were investigated by insitu IR microspectroscopy. All metal substituted samples showed IR bands which can be assigned to the bridging hydroxy groups. Their acid strength determined from the perturbation of the bands by sorption of benzene decreased in the sequence SAPO-5>MgAPO-5> BeAPO-5>CoAPO-5. Sorption of strong bases (ammonia and pyridine) induced the cleavage of at least one bond between the framework metal cation and oxygen at the bridging hydroxy groups leading to strong coordinative interactions of the base molecules with the framework metal cations.