Investigations on the CoAPO-36 molecular sieve

Highly crystalline cobalt aluminophosphates of type 36 have been synthesized and characterized. Investigations on the thermal decomposition of Pr₃N-CoAPO-36 and the surface, sorption, acid strength distribution, acidic and catalytic properties of CoAPO-36 were carried out. The XPS analysis indicated that the concentration of cobalt was higher in the bulk of the material than on the surface. The surface of the cobalt aluminophosphate is aluminium rich. The number of strong acid sites is higher on the CoAPO-36 than on CoAPO-5, MAPO-5 and ZAPO-36. The catalytic activities of CoAPO-5, MAPO-5, ZAPO-36, CoAPO-36, MAPO-36 and MAPSO-36 in the 3-methylpentane ando-xylene conversion reactions were compared. The catalytic turnover rate per framework substituted atom in the conversion ofo-xylene for CoAPO-36 is higher than for the MAPO-5, CoAPO-5, ZAPO-36 and MAPSO-36. In the ethylbenzene conversion reaction, the deactivation of the cobalt aluminophosphate and magnesium aluminophosphates of type 36 were studied.     

Selective ion-exchange in MAPO-36 and its application in toluene disproportionation

MAPO-36 was synthesized hydrothermally by isomorphic substitution of Mg²⁺ in the framework of AlPO-36 and ion-exchanged with Fe³⁺, Zn²⁺, La³⁺ and Ce³⁺ by wet method. The materials were characterized by XRD, TGA, TPD (ammonia) and SEM-EDX. XRD revealed absence of structural degradation after ion-exchange. TPD (ammonia) showed selective ion-exchange of strong acid sites in ion-exchanged MAPO-36. The weight loss around 550 °C in TGA for Fe, La and CeMAPO-36 suggested conversion of M(OH)₂⁺ to MO⁺. Toluene disproportionation was carried over all catalysts in which diphenyl methane derivative was suggested to be the principle intermediate in the formation of p-xylene and other products. The time on stream study showed exclusive formation of p-xylene after 6 h.     

Dinitrogen as a probe of acid sites of zeolites

Nitrogen adsorption on H-ZSM-5 and H-Y zeolites at low temperatures were studied by in situ FT-IR spectroscopy. For each zeolite, two absorption bands were observed at around 2334 and 2352 cm^–1 in thev(NN) region and were assigned to thev(NN) mode of dinitrogen species adsorbed on Brønsted and Lewis acid sites of the zeolites, respectively. These results and previous results for H-mordenite suggest that dinitrogen serves as a probe of acid sites and its advantages as probe are discussed.     

A high‐resolution solid‐state NMR study on nano‐structured HZSM‐5 zeolite

Variations in the structure and acidity properties of HZSM‐5 zeolites with reduction in crystal sizes down to nanoscale (less than 100 nm) have been investigated by XRD, TEM and solid‐state NMR with a system capable of in situ sample pretreatment. As evidenced by a combination of ²⁷Al MAS NMR, ²⁹Si MAS, CP/MAS NMR and ¹H MAS NMR techniques, the downsize of the zeolite crystal leads to an obvious line broadening of the ²⁷Al, ²⁹Si MAS NMR spectrum, an increasing of the silanol concentration on the external surface, and a pronounced alteration of the acidity distribution between the external and internal surfaces of the zeolite. In a HZSM‐5 zeolite with an average size at about 70 nm, the nonacidic hydroxyl groups (silanols) are about 14% with respect to the total amount of Si, while only 4% of such hydroxyl groups exist in the same kind of zeolite at 1000 nm crystal size. The result of ¹H MAS NMR obtained using Fluorinert^® FC‐43 (perfluorotributyl amine) as a probe molecule demonstrates that most of the silanols are located on the external surface of the zeolite. Moreover, the concentration of Brønsted acid sites on the external surface of the nano‐structured zeolite appears to be distinctly higher than that of the microsized zeolite.     

An NMR study of acid sites on sulfated-zirconia catalysts using trimethylphosphine as a probe

Concentrations of Brønsted and Lewis acid sites on sulfated-zirconia catalysts were determined using the³¹P MAS NMR spectra of adsorbed trimethylphospine. A sample that had been calcined and exposed to air for a long period exhibited only Brønsted acidity; however, treatment of the sample at progressively higher temperatures resulted in the development of at least three types of Lewis acidity, along with a decrease in the concentration of Brønsted acid sites. In a related study the activity of these catalysts for the alkylation of isobutane with 2-butene was determined. The aged catalyst was inactive, but activation of the material at 100°C resulted in the most active catalyst. Thermal treatment at higher temperatures resulted in a loss in activity which paralleled the decrease in the Brønsted acid sites. These results are consistent with a model in which strong Brønsted acidity is a result of the interaction between bisulfate groups and adjacent Lewis acid sites.     

Thermal Analysis of Magnesium/Perfluoropolyether Pyrolants

Exothermic reactions between metals and fluorinated polymers are found in a variety of energetic materials, including reactive binder systems and the Magnesium‐Teflon‐Viton incendiary composition. This paper describes the reactions between a high molecular weight perfluoropolyether, Fomblin Y 140/13, and magnesium in a variety of morphologies including μm‐scale powders and nano‐scale layered films. Using Temperature Programmed Desorption and Temperature Programmed Reaction we have found that the magnesium‐perfluoropolyether interaction is characterized by: (1) competition between Fomblin decomposition and desorption, and (2) magnesium passivation by the formation of magnesium fluoride. Differential Scanning Calorimetry measurements establish a lower‐bound estimate of the specific reaction energy of 9.2 kJ g⁻¹. High molecular weight Fomblin (6500 amu) undergoes a competitive reaction/desorption process with desorption occurring at 550 K and decomposition at 610 K. Decomposition becomes more favorable relative to desorption for higher heating rates and thicker films. Perfluoropolyethers produce several characteristic ions in the 70 eV election ionization mass spectra, with the CF₃⁺ ion being the most abundant ion observed during both the molecular desorption and decomposition. Larger fragment ions with masses of 235 and 285 amu are observed in relatively high concentrations during desorption and low concentrations during decomposition. The reaction between magnesium and Fomblin begins at 400 K, producing CF₃⁺, CO⁺, and C₂F₅⁺ in the electron ionization mass spectrum. We propose that these reactions form a passivating layer of magnesium fluoride that protects the remaining metal as it approaches the magnesium melting point. Most of the reaction takes place at 800 K and above when the magnesium fluoride film ruptures.     

31P and119Sn high resolution solid state CP/MAS NMR study of Al2O3-SnO2 systems

A series of Al₂O₃-SnO₂ catalysts with the mole ratio of Al₂O₃ to SnO₂ equal to 1:1, 1 0.5, 1 0.1, 1 0.05 and 1 0.01 were characterized by³¹P NMR of adsorbed trimethyl phosphine (TMP) and¹¹⁹Sn MAS NMR spectroscopy. It was found from³¹P NMR that no Brønsted acid sites exist in these samples. Pure SnO₂ shows two different types of Lewis acid sites; in the mixed oxide samples a Lewis peak characteristic of pure Al₂O₃ is always seen, together with either one or two other Lewis peaks, depending on the Sn concentration.¹¹⁹Sn CP/MAS NMR spectra of the highest Sn-content sample show one narrow line at –603 ppm superimposed on a very broad line, indicating a strong interaction between Al and Sn oxides.     

Identification and Influence of Acidity on Alkylation of Phenol with Propylene over ZSM-5

The alkylation of phenol with propylene has been studied over several H-ZSM-5s with different Si/Al ratios and Cs⁺-ion-exchanged H-ZSM-5s at temperature range 373–623°C. Both O- and C-alkylation, which were closely dependent on the reaction temperature and acidity of the catalysts, were observed. O-alkylated compound is found to be formed preferably at temperature lower than 250°C and over Cs⁺-ion-exchanged H-ZSM-5s. However, at higher temperature, only C-alkylation is observed. The acidic properties of the zeolites were characterized by solid-state ³¹P MAS-NMR of the probe molecule trimethylphosphine oxide and NH₃-TPD (temperature-programmed desorption) and it is suggested that in the case of C-alkylation, moderate acid sites are responsible for the formation of para-isopropylphenol, while ortho-isopropylphenol is favorable for weak acid sites.     

Experimental and DFT studies of N2O decomposition over bare and Co-doped magnesium oxide—insights into the role of active sites topology in dry and wet conditions

Isothermal and TPSR studies of deN₂O reaction were performed over model MgO, and Co–MgO systems. Complementary experiments such as O₂-TPD, H₂O-TPD and IR, supported by DFT molecular modeling were carried out to elucidate the role of topological irregularities such as corners, steps, edges on the principal reaction events in dry and wet environment. The hydroxyl groups produced upon water adsorption were used for selective blocking of the active sites. Topological aspect of the surface reactivity was discussed in terms of the energetics of reaction steps and the relative stability and mobility of reaction intermediates (O₂²⁻) and inhibiting (OH⁻_surf) species. Whereas the most active corner sites are depleted by oxygen, produced in the course of N₂O decomposition, the edge and step sites could be eliminated only by more strongly adsorbed water. In steady state conditions the reaction turns over mainly on terrace sites and is moisture insensitive. Upon introduction of Co²⁺ ions to magnesium oxide the N₂O conversion curve is shifted by 200 °C towards lower temperatures, due to the change in the N₂O activation step (from anionic redox to cationic redox). However, at such thermal conditions the terrace sites remain still hydroxylated, and the overall reaction rate is clearly deteriorated by water present in the feed.     

NO and N2O emissions from savanna soils following the first simulated rains of the season

Data on the emissions of oxides of nitrogen from the soil during the early part of the wet season are reported for nutrient-rich and nutrient-poor sandy soils at Nylsvley, South Africa. The emissions of NO_x and N₂O following the first wetting event of the season are elevated relative to subsequent events. The observed high emission rates (76 ng N-NO m^-2 s^-1) are partially attributed to the sandiness of the soil, which permits NO to diffuse out of the soil rapidly. The pulse of high emissions following wetting is maintained for approximately 72 hours, thereafter continuing at around 20 ng NO m^-2 s^-1 while the soil remains moist. The initial pulse is suggested to be due to the accumulation of a substrate pool during the dry period, coupled with an inability of plants and microbes to use it effectively during the first few days after wetting. There were no significant differences in the peak or subsequent emission rates for either NO or N₂O between two sites of differing nitrogen mineralisation potentials. N₂O emissions averaged 8% of NO_x emissions. The enhanced emissions of NO_x which follow the first wetting after a prolonged dry period do not make a very large contribution to the annual gaseous N emission budget, but could be a significant contributor to the high tropospheric ozone levels observed over southern Africa in springtime.     

Analysis of oxidation states of vanadium in vanadia–titania catalysts by the IR spectra of adsorbed NO

Adsorption of NO on vanadia–titania samples pre-subjected to different reduction treatments has been studied by FTIR spectroscopy. When the NO adsorption is performed at 85 K on oxidized samples, antisymmetric NONO species, typical for V⁵⁺ sites, are detected and characterized by bands at 1779 and 1686 cm⁻¹. At ambient temperature, however, adsorption is negligible and only with time reactive adsorption occurs producing NO⁺ (2120 cm⁻¹), nitro/nitrato species (bands in the 1650–1100 cm⁻¹ region) and weakly adsorbed NO (broad band at 1915 cm⁻¹). Adsorption of NO at ambient temperature on reduced samples results in the formation of two types of species: (i) V⁴⁺(NO)₂ dinitrosyls characterized by ν_s(NO) and ν_as(NO) at 1903–1880 and 1769–1753 cm⁻¹, respectively, and (ii) V³⁺(NO)₂ complexes, which give rise to ν_s(NO) at 1834–1822 cm⁻¹ and ν_as(NO) at 1697–1685 cm⁻¹. At low temperature the dinitrosyls are transformed into species in which more than one (NO)₂ dimer is attached to one cationic site. Addition of O₂ to NO, preadsorbed on reduced vanadia–titania samples, results in a fast oxidation of the V³⁺(NO)₂ species, whereas the V⁴⁺(NO)₂ complexes are more stable and do not disappear completely in the presence of oxygen. The results obtained suggest that NO is a convenient probe molecule for the analysis of the oxidation state of vanadium in vanadia–titania catalysts. To prevent oxidation of reduced vanadium sites, low equilibrium pressures of NO and registration of the IR spectrum soon after the NO admission are recommended. This revised version was published online in August 2006 with corrections to the Cover Date.     

Alkali binding by magnesium silicate hydrates

The binding of Na⁺, K⁺, and Li⁺ by magnesium silicate hydrate (M–S–H) was investigated in batch sorption experiments. Sorption isotherms and cation exchange measurements indicated the binding of alkalis in cation exchange sites compensating the negative surface charge of M–S–H. Higher pH values led to further deprotonation of the silanol groups and a higher alkali uptake by M–S–H. No significant incorporation of alkalis in the main silica or magnesium oxide sheets was observed. However, the silica sheets were less polymerized in the presence of higher alkali hydroxide concentrations.     

Interactions between fullerene(C60) and poly(ethylene oxide) in their complexes as revealed by high-resolution solid-state C NMR spectroscopy

A series of poly(ethylene oxide) (PEO)/fullerene(C₆₀) complexes are prepared by lyophilization. The intermolecular interaction and molecular motion in the complex are investigated by solid-state ¹³C NMR spectroscopy. An intense C₆₀ signal due to the intermolecular cross-polarization is observed in the ¹³C CP/MAS spectra of the complex samples, indicating a high degree of dispersion of C₆₀s in the complexes. By measuring the ¹³C spin-lattice relaxation times and ¹H transverse relaxation times of the complex sample and by comparing the static ¹³C spectrum of the pure C₆₀ sample with that of the complex sample, it is demonstrated that there exist n-π interactions between the n-orbitals of the PEO ether oxygen and the π-system of C₆₀. The C₆₀ molecules act as physical cross-links in the amorphous region of PEO, which greatly inhibit the mobility of the surrounding PEO chains, while the rapid isotropic rotation of C₆₀ molecules is also reduced to some extent due to the interactions with the polymer chains.     

Acidic properties of vanadium oxide on magnesium fluoride

Vanadium oxides supported on magnesium fluoride obtained by the impregnation and the precipitation-deposition method have been studied by IR spectroscopy. The adsorption of pyridine indicated the presence of Lewis acid centers on the impregnated samples, whereas on the samples prepared by precipitation both Lewis and Brønsted centers have been observed. From the adsorption coefficients of the 1540 cm^–1 (Brønsted site) and 1445 cm^–1 (Lewis site) bands the numbers of both sites have been estimated. The hydroxyl groups bound in the chains of the (VO₃)_n tetrahedra or those in the trigonal pyramids VO₅ are believed to be responsible for the protonic centers, whereas the aprotonic centers occur at the ionic vanadium with coordinatively unsaturated spheres.     

A31P nuclear magnetic resonance investigation of acyl group transfer from phosphatidylcholine to yield lysophosphatidylcholine in human plasma

³¹P nuclear magnetic resonance (NMR) spectroscopy was used to measure the rate of acyl transfer from phosphatidylcholine (lecithin, PC) in whole plasma and in high density lipoprotein (HDL). Spectral deconvolution was used to resolve overlapping resonances in the³¹P NMR spectra of the phospholipids. Mean values of the acyl group transfer rates from PC in plasma and HDL were 36 μmol L⁻¹h⁻¹ and 19 μmol L⁻¹h⁻¹, respectively. The reciprocal nature of the decrease in the spectral peak intensities of PC, compared to the increase in the intensities of the lysolecithin (lysoPC) peaks, suggested a substrate/product relationship consistent with the action of lecithin:cholesterol acyltransferase (LCAT), the enzyme responsible for the esterification of free cholesterol in plasma. LCAT involvement was confirmed by measuring the cholesterol esterification rate based on the¹³C NMR spectra obtained on lipid extracts from plasma that had been incubated at 37°C. Within experimental error, the rate of lysoPC formation in plasma was shown to be equal to that of cholesteryl ester formation.     

The activation of O2 and the oxidative dehydrogenation of C2H6 over SmOF catalyst

The activation of O₂ over SmOF was studied by in situ laser Raman spectrometry and temperature programmed desorption (TPD). When the hydrogen- and helium-treated (1 h for each gas at 973 K) SmOF sample was cooled to 303 K in oxygen, Raman bands which correspond to the existence of O ₂ ^2– , O ₂ ^n– (2 >n > 1), O ₂ ^– and O ₂ ^- (1 > > 0) species were observed. From 303 to 973 K, there was no O₂ desorption but the Raman bands observed at 303 K reduced in intensity and vanished completely at 973 K, even though the sample was under an atmosphere of oxygen. We suggest that as the sample temperature increased, dioxygen species were converted to mono-oxygen species such as O^– which were undetectable by Raman spectrometry. O₂ desorption occurred above 973 K, giving a TPD-peak at 1095 K. When C₂ H₆ was pulsed over the sample pretreated with oxygen and helium at 973 K, C₂H₄ selectivity was 91.8%. We conclude that the mono-oxygen species is responsible for the oxidative dehydrogenation of ethane to ethene.     

FTIR study of low-temperature CO adsorption on Cu/silicalite-1

Adsorption of CO on Cu/silicalite reveals the existence of Cu⁺ sites similar in properties to the Cu⁺ cations exchanged in Cu–ZSM-5. In both cases Cu⁺(CO)₂ dicarbonyls are formed at ambient temperature and under CO equilibrium pressure. These species are converted into monocarbonyls after evacuation. At low temperature, as with Cu–ZSM-5, tricarbonyls are formed on part of the Cu⁺ sites on Cu/silicalite-1. However, the polycarbonyls on the Cu/silicalite-1 sample are less stable as compared to Cu–ZSM-5. In addition, a fraction of Cu⁺ sites rather typical for oxide-supported copper is found with Cu/silicalite-1. The location of the different copper cations in Cu/silicalite-1 is discussed.     

Thermally stimulated current study on flexible siloxane-modified epoxy sheets

Copolymers prepared from the epoxy resin of ALBIFLEX and PMPS oligomer were evaluated with IR, ¹H-and ¹³C NMRs for structural determinations. The flexible epoxy resin as a model compound was examined by TSC with variations on the operational parameters t_p and V_p. Transitions denoted as T_g,δ, T_g,γ, T_g,β, T_g,α and T_g,ρ in increasing order of temperature were observed for these copolymers. As an example, ESAF-7430 copolymer showed these transitions at −138, −84, 26, 87, and 110 °C, respectively. Each transition except the T_g,ρ at 110 °C, has been correlated to a segment in the copolymeric structure. The result indicates that TSC provides clear molecular transitions at the temperature of the transition. The molecular transition of the grafted PMPS-siloxane segment in an epoxy matrix appeared at the −45 °C region as a broad, nearly flat peak as it filled the concave part of the TSC curve of the unmodified resin. The flexibility that the modified copolymers retained may be attributed to these sub-T_g transitions as observed in the TSC spectrum.     

Changes upon heating of the distribution of Al,P and Si atoms in the SAPO-37 molecular sieve

A drastic change in the environment of Si atoms in SAPO-37 after heating at 1173 K is seen in the²⁹Si MAS NMR spectrum. This suggests a modification of the location of Si, Al and P atoms. Three main phases would coexist in large amounts in the faujasite structure, the SAPO-37 originating phase, pure SiO₄ islands and an aluminosilicate phase comparable to Si-Al faujasite.     

Synthesis and characterization of nanocrystallite ZnCrFeO4

Nanocrystallites of spinel ZnCrFeO₄ were synthesized through sol–gel method, and were characterized by X-ray diffractometer (XRD), transmission electron microscopy (TEM), SQUID magnetometer and Mössbauer spectrometer. It is found that the nanocrystallites are spherical and show antiferromagnetism with a higher T_N of 14 K for the sample with diameter of 14 nm and larger magnetic susceptibility than the bulk sample. These are associated with some A ions (Zn²⁺) and B ions (Fe³⁺, Cr³⁺) jumped to each other sites, which therefore enhances both the A–B interaction and B–B interaction. The Mössbauer spectrum of the nanomaterial is different from that of the bulk sample. Additional sextet appears in the Mössbauer spectrum of the nanomaterial except for a doublet. The appearance of the sextet suggests the magnetically ordered state between Fe³⁺ ions at A and B sites.