Sandia octahedral molecular sieves (SOMS) is an isostructural, variable composition class of ion exchangers with the general formula Na(2)Nb(2-x)M(IV)(x)O (6-x)(OH)(x).H(2)O (M(IV) = Ti, Zr; x = 0.04-0.40) where up to 20% of the framework Nb(V) can be substituted with Ti(IV) or Zr(IV). This class of molecular sieves is easily converted to perovskite through low-temperature heat treatment (500-600 degrees C). This report provides a detailed account of how the charge imbalance of this Nb(V)-M(IV) substitution is compensated. X-ray powder diffraction with Rietveld refinement, infrared spectroscopy, thermogravimetric analysis, (23)Na MAS NMR, and (1)H MAS NMR were used to determine how the framework anionic charge is cation-balanced over a range of framework compositions. All spectroscopic evidence indicated a proton addition for each M(IV) substitution. Evidences for variable proton content included (1) increasing OH observed by (1)H MAS NMR with increasing M(IV) substitution, (2) increased infrared band broadening indicating increased H-bonding with increasing M(IV) substitution, (3) increased TGA weight loss (due to increased OH content) with increasing M(IV) substitution, (4) no variance in population on the sodium sites (indicated by Rietveld refinement) with variable composition, and (5) no change in the (23)Na MAS NMR spectra with variable composition. Also observed by infrared spectroscopy and (23)Na MAS NMR was increased disorder on the Nb(V)/M(IV) framework sites with increasing M(IV) substitution, evidenced by broadening of these spectral features. These spectroscopic studies, along with ion exchange experiments, also revealed the effect of the Nb(V)/M(IV) framework substitution on materials properties. Namely, the temperature of conversion to NaNb(1-x)M(IV)(x)O(3) (M = Ti, Zr) perovskite increased with increasing Ti in the framework and decreased with increasing Zr in the framework. This suggested that Ti stabilizes the SOMS framework and Zr destabilizes the SOMS framework. Finally, comparing ion exchange properties of a SOMS material with minimal (2%) Ti to a SOMS material with maximum (20%) Ti revealed the divalent cation selectivity of these materials which was reported previously is a function of the M(IV) substitution in the framework. A thorough investigation of this class of SOMS materials has revealed the importance of understanding the influence of heterovalent substitutions in microporous frameworks on material properties.

No abstract

No abstract

The passivation process was carried out and monitored employing a three electrode electrochemical glass cell. Surface analysis using grazing incidence X-ray diffraction (GIXRD) elucidated that trace magnetite in the dominant siderite (FeCO 3 ) was responsible for the passivation. Transmission electron microscopy (TEM) with the energy dispersive X-ray fluorescence (EDX) technique was used to determine the structure of the passive layer and confirm its chemistry. A passive phase tens of nanometers thick was observed beneath iron carbonate scale and at the crystal boundaries. Scanning transmission electron microscopy (STEM)/EDX profiles suggest that this phase is not a carbonate containing compound since only oxygen and iron were observed. This indicates that the possible chemical compound for the passive phase is an iron oxide, agreeing with the previous GIXRD surface analysis. Fe 3 O 4 as a passive film was confirmed.

In this Communication, we report the first example of a network structure composed of vanadosilicate clusters. We utilized hydrothermal conditions to synthesize a polyoxovanadogermanate (POVG): (C4H14N2)4[V14O44(GeOH)8].6H2O. By substituting SiO2 for GeO2 in the synthesis, a framework solid, H4V18O46(SiO)8C4H12N2)4.(H2O)] .4H2O, is formed in which isostructural vanadosilicate clusters are linked by five-coordinate vanadium with a (VO)O2N2 environment. The charge-compensating organic amine, 1,4-diaminobutane, in the POVG is covalently bonded to the linking vanadium polyhedra in the framework solid.

No abstract

Occlusion of benzene in NaZSM-5 zeolite is investigated using in situ FTIR spectroscopy as a function of substitution of Na+ with group IIA cations. At least four pairs of overlapping vibrational bands were observed in the region of out-of-plane C−H bending vibrations (2000−1800 cm-1) on adsorption of benzene in NaZSM-5 at room temperature. Whereas two pairs of these bands, e.g., one pair at around 2007 and 1986 cm-1 and the other at 1969 and 1956 cm-1, correspond to the 1960 cm-1 band of liquid benzene, the other two pairs, e.g., at 1874 and 1852 cm-1 and 1831 and 1810 cm-1, appear in place of the 1815 cm-1 band of liquid benzene in this region. No measurable difference was observed in the frequencies of these bands for adsorption in cation-exchanged samples, suggesting that any specific interaction between cations and benzene molecules is small compared to the effect of benzene−benzene interaction. These multiple bands are therefore attributed to the existence of at least two distinct clustered states of benzene, localized at intersections and in the straight channels of NaZSM-5, respectively. While the frequency of these bands remained unchanged, the intensity of the lower frequency side pair (i.e., 1969, 1956 cm-1 and 1831 and 1810 cm-1) was found to be very sensitive to the nature of the charge-balancing cation and followed a trend NaZSM5 < CaZSM5 > SrZSM5 > BaZSM5, similar to that followed by the pore volume of exchanged samples. These two pairs of bands are therefore identified with the benzene clusters encapsulated in straight zeolitic channels where most of the balancing cations are located. Dose-dependent measurements have shown that such benzene clusters may form at loading as low as ∼1.6 molecules/uc; when a larger fraction is located at intersection sites and at the same time a small fraction also exists in the straight or sinusoidal channels. The concentration in the later locations grows with the increasing benzene loading. Considering these results and in view of the fact that no frequency shift or band splitting was observed in the in-plane C−H/C−C and fundamental ν19 stretching vibrations of adsorbed benzene, we infer that the benzene molecules are packed side by side with their planes parallel to the zeolite channel, the intermolecular interaction occurring through π-electron cloud.

No abstract