Aluminum Sites Research Articles

Synthesis, characterization and catalytic activity of single site, Lewis acidic aluminosilicates

A general synthetic strategy for the preparation of atomically dispersed, well-defined Lewis acidic aluminum sites in silicate matrices is described and illustrated. These amorphous matrices are composed of the spherosilicate cubes (Si8O20) linked together by combinations of aluminum and siloxane groups. The connectivity of the catalytically active aluminum centers to the surrounding silicate matrix can be varied by choice of aluminum precursor. “Extraframework” 3-connected Al⟵Py and Al⟵THF sites are obtained from the corresponding AlCl3·L complexes and 4-connected “framework” sites from Y[AlCl4] (Y = Lutidinium, [Bu4N]+). The aluminum sites in these porous silicates are characterized by gravimetry, IR and multinuclear SSNMR spectroscopies as well as ligand exchange reactions. Their catalytic activities as Lewis acids are investigated for the aminolysis of styrene oxide by aniline. These single site aluminosilicate catalysts exhibit activities that are comparable to or better than analogous, well-known zeolite and amorphous aluminosilicate materials. Their activities are dependent on the connectivity of aluminum to the surrounding silicate matrix as well as the length and flexibility of secondary linkers in the matrix.

Chemical poisoning by zinc and phosphorous of Pt/Ba/Al2O3 NOx storage catalysts

The effects of phosphorous and zinc on the performance of Pt/Ba/Al2O3 catalysts were investigated through wet impregnation of ammonium phosphate and zinc acetate aqueous solutions. Six different sample combinations were studied; 1 wt-% P, 1 wt-% Zn, 1 wt-% P with 1 wt-% Zn, 2 wt-% P, 2 wt-% Zn, 2 wt-% P with 2 wt-% Zn. NOx storage and reduction (NSR) activity and NO2 temperature programmed desorption (TPD) profiles were measured before and after impregnation of P and Zn. Samples containing P performed significantly worse than samples only containing Zn in both NSR activity and TPD measurements. The increased NOx slip during lean phase in activity measurements for P-poisoned samples is mainly related to an increased slip of NO2. This was found for both NO and NO2 in the gas feed during lean phase and suggests that it is mainly the storage component that is poisoned and not the noble metal. Furthermore, the combination (1 wt-% P and 1 wt-% Zn) proved to result in slightly worse performance than only 1 wt-% P, however this was not the case for samples containing 2 wt-%, where the addition of zinc reduced the negative effect of phosphorous. Measurements from NO2-TPD experiments showed that NOx release at low temperature was not affected by the addition of P, while desorption in the temperature range 425–475 °C was significantly reduced. It can therefore be concluded that the poisoning mainly is related to barium NOx storage sites and not to alumina sites. Moreover, X-ray diffraction measurements indicate that some of the barium species are affected by phosphorous. Images from scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDX) mapping were in line with the results seen in both the activity tests and NO2-TPD experiments. Phosphorous was concentrated at the same position as barium in the observed images, whereas zinc was more evenly distributed over the surface. For the sample with both 2 wt-% P and 2 wt-% Zn, X-ray photoelectron spectroscopy measurements indicate that Zn and P have a low interaction and this suggests that most of the zinc and phosphorous are separated. However, STEM-EDX showed agglomerates of some zinc and phosphorous, which could be zinc phosphates. This is a plausible explanation of the decreased deactivation observed after introducing 2 wt-% Zn compared to the sample only containing 2 wt-% P.

Short contact with nickel causes allergic contact dermatitis

Allergic contact dermatitis is a common type of eczema that causes inflammation when skin comes into contact with certain substances. Although it is well accepted that long term skin contact can cause nickel allergy and dermatitis, the significance of relatively short and repeated nickel exposure in relation to induction and elicitation (i.e. first triggering) of nickel allergy has been less explored, and it has been proposed that short skin contact with nickel is safe. A study conducted in Denmark and Sweden attempted to test this proposition by examining whether repeated skin contact with nickel over short time periods would elicit allergic contact dermatitis. 16 adults who were known to have nickel allergy were tested along with a further 10 non‐allergic control subjects (a control subject is a healthy individual used for comparison). The subjects were exposed to nickel and aluminium discs on each forearm and each earlobe for 3 x 10 minutes. Also, one arm would be treated for 24 hours beforehand with 0.5% sodium lauryl sulfate (SLS) under occlusion (meaning the solution was applied then wrapped up with plastic wrap or cloth). SLS is added to bath products and detergents to make them foam and is a common cause of skin reactions. The purpose of this was to monitor differences in reactions to nickel in skin that was either normal or already irritated (by the SLS). Each nickel and aluminium site was evaluated and blood flow measured, as increased blood flow can show that skin is reacting to nickel. 62.5% of nickel allergic subjects reacted to nickel on the SLS treated arm. The nickel exposed skin also caused significantly more blood flow than aluminium exposed skin. No change in reactivity or blood flow was seen in any non‐SLS nickel exposed arms in nickel allergic patients, or in any control subjects. This study shows that relatively short repeated contact with nickel can elicit allergic contact dermatitis in irritated skin in people who already have a nickel allergy. The results support the restrictions in current nickel regulation.

Morphological and Structural Analyses in Carbonated Magnesium-Aluminum Hydrotalcites Co-Substituted with Iron III

Magnesium-aluminum hydrotalcites can be co-substituted with others trivalent cations, such as iron III in aluminum site, which can be a promising way to modify the properties of that synthetic adsorptive material. In the present work, hydrotalcite containing 5 mol% iron III in co-substitution to the aluminum was prepared by precipitation process and compared with no co-substituted sample along the temperature of calcination at 100 and 500 oC for 4 hours. The calcined samples were characterized by simultaneous TG/DTA, X-ray Diffraction and nitrogen adsorption-desorption techniques. The iron (III) insertion showed positive results in order to provide more stable structure against high temperatures of calcination, which was observed by lesser structural decomposition of rhombohedral hydrotalcite and a more mesoporous structure.

The Effect of Iron (III) Co-Insertion in Magnesium-Aluminum Hydrotalcites Obtained by Precipitation Method at pH 11

Hydrotalcite are anionic clay material presenting LDH arrangement and high porosity and specific areas, which make it a good adsorbent for pollutant species in water. Besides that, that material type can be used as catalyst or catalyst support in several industrial processes. The most common compositions is based on metallic mix hydroxide with high content of magnesium, but their adsorptive properties arise from aluminum replacement in layer structure. The present work presents the synthesis of carbonated magnesium-aluminum hydrotalcite through the precipitation method in order to investigate the co-insertion of iron (III) in aluminum site. It was found the iron (III) co-inserted samples obtained at 100 and 200 oC for 4 hours present no substantial harming in relation to the common magnesium-aluminum composition. All of the samples presented high porosity and specific area, becoming an alternative anionic adsorptive.

Enhanced stability of Pd/Al2O3 during aqueous oxidation reaction via SiH4 treatment

γ-Al2O3 is widely present as a support in catalytic application. However, the transformation of γ-Al2O3 into the undesired hydrated boehmite (γ-AlOOH) under the aqueous reaction conditions usually results in an irreversible inactivation of supported Al2O3 catalysts. Toward suppressing the hydration of γ-Al2O3 in the process of catalytic reactions, herein we have devised a new strategy by exploring the SiH4 treatment for successful preparation of Si–Pd/Al2O3 catalysts. SiH4 treatment enables a significant improvement in the poor stability of Pd/Al2O3 for selective oxidation of toluene because SiH4 could effectively anchor palladium nanoparticles and inhibit the formation of boehmite by reacting with unsaturated aluminum sites to reduce the intensity of Lewis acid sites on the surface of γ-Al2O3. Pd/Al2O3 pretreated with SiH4, that is Si–Pd/Al2O3, provides high catalytic activity and stability in the aqueous oxidation of toluene even at high temperature. Moreover, Si–Pd/Al2O3 is reusable without obvious loss of the catalytic activity and selectivity, compared to Pd/Al2O3.

The Nature of Chemical Bonding in Lewis Adducts as Reflected by 27Al NMR Quadrupolar Coupling Constant: Combined Solid-State NMR and Quantum Chemical Approach.

Lewis acids and Lewis adducts are widely used in the chemical industry because of their high catalytic activity. Their precise geometrical description and understanding of their electronic structure are a crucial step for targeted synthesis and specific use. Herein, we present an experimental/computational strategy based on a solid-state NMR crystallographic approach allowing for detailed structural characterization of a wide range of organoaluminum compounds considerably differing in their chemical constitution. In particular, we focus on the precise measurement and subsequent quantum-chemical analysis of many different 27Al NMR resonances in the extremely broad range of quadrupolar coupling constants from 1 to 50 MHz. In this regard, we have optimized an experimental strategy combining a range of static as well as magic angle spinning experiments allowing reliable detection of the entire set of aluminum sites present in trimesitylaluminum (AlMes3) reaction products. In this way, we have spectroscopically resolved six different products in the resulting polycrystalline mixture. All 27Al NMR resonances are precisely recorded and comprehensively analyzed by a quantum-chemical approach. Interestingly, in some cases the recorded 27Al solid-state NMR spectra show unexpected quadrupolar coupling constant values reaching up to ca. 30 MHz, which are attributed to tetra-coordinated aluminum species (Lewis adducts with trigonal pyramidal geometry). The cause of this unusual behavior is explored by analyzing the natural bond orbitals and complexation energies. The linear correlation between the quadrupolar coupling constant value and the nature of bonds in the Lewis adducts is revealed. Moreover, the 27Al NMR data are shown to be sensitive to the geometry of the tetra-coordinated organoaluminum species. Our findings thus provide a viable approach for the direct identification of Lewis acids and Lewis adducts, not only in the investigated multicomponent organoaluminum compounds but also in inorganic zeolites featuring catalytically active trigonal (AlIII) and strongly perturbed AlIV sites.

Impact of Al sites on the methane co-aromatization with alkanes over Zn/HZSM-5

The presented work investigates the co-aromatization of methane and propane over Zn/HZSM-5 catalysts. The impact of Al sites, such as framework and extra-framework aluminum (EFAL) sites, on the methane participation pathway during the co-aromatization reaction is explored. The Al sites in the catalysts are manipulated by using ZSM-5 support materials with variable SiO2/Al2O3 molar ratios. Post-synthetic treatments, such as the ammonium hexafluorosilicate (AHFS) treatment to remove EFAL and the steam treatment to convert some of the framework Al sites to EFAL sites, are also employed to prepare catalysts and evaluate the contribution of these Al sites to the methane participation pathway. A higher concentration of Al in the catalysts would facilitate the methane participation in the phenyl ring formation. An average carbon number increment of 0.93, a substitution index reduction of 0.028, and an enhanced aromatic carbon fraction by 0.036 among the product molecules are witnessed over the Zn/HZSM-5 (23:1) catalyst during the co-aromatization reaction. This phenomenon is further enhanced when some of the framework Al atoms are converted to the EFAL sites by the steam treatment. The changes of substitution index and aromatic carbon fraction are expanded to 0.045 and 0.055. When the total Al concentration is low in the catalyst, however, the increased EFAL site number at the cost of framework Al sites does not improve methane participation, indicating that the framework Al sites are also crucial for the methane participation. The chemical environment of the Al sites in the catalysts is probed by 27Al solid-state NMR and XAS studies. The NH3-TPD and DRIFTS investigations of the catalysts demonstrate that the framework Al sites and the EFAL sites are closely related to the Brönsted and Lewis acid sites, respectively. The TEM and XRD studies reveal the intact crystalline structure of the catalysts upon the AHFS and steam treatments. The impact of the Al environments on the methane participation pathway in the co-aromatization reaction may be due to the acid properties of the catalysts.

Nitric Oxide Adsorption in MIL-100(Al) MOF Studied by Solid-State NMR

Adsorption of nitric oxide (NO) in the metal–organic framework (MOF) MIL-100(Al) is studied by solid-state NMR. Owing to a modified synthesis, no extraframework benzenetricarboxylate is present on the cost of a small amount of extraframework Al(OH)3 as evident from 27Al, 1H, as well as heteronuclear correlation spectra. Five-coordinated aluminum sites represent about 50% of the aluminum in a dehydrated sample, which remain open for adsorption. With increasing NO loading, a decrease of five-coordinated aluminum with a subsequent increase of six-coordinated aluminum site intensity is found. Additionally, 1H spin–lattice relaxation time T1 is decreasing with an increasing amount of NO, which also supports the NO interaction with the MOF because of the paramagnetism of NO. Fourier-transform infrared spectroscopy data further hint at Al–NO interactions.

Inelastic Neutron Scattering Study of the Aluminum and Brønsted Site Location in Aluminosilicate LTA Zeolites

Inelastic neutron scattering (INS) has been employed to determine the location of aluminum and Bronsted acid sites in an LTA-5 sample (Si/Al = 5) with few defects of connectivity. A large number of Al and Bronsted distributions were computationally generated, and a selection of nine distributions were found to be in agreement with the 29Si NMR spectrum of the LTA sample. From the nine subsequent periodic density functional theory calculations, one of the models shows a good agreement with the INS spectrum. The suggested distribution shows Al atoms to be homogeneously distributed in the framework and protons distributed relatively far from each other. A CIF file containing the LTA-5 distribution of Al and H matching the experiments is given as the result.

Relation of Catalytic Performance to the Aluminum Siting of Acidic Zeolites in the Conversion of Methanol to Olefins, Viewed via a Comparison between ZSM-5 and ZSM-11

ZSM-5 and ZSM-11 zeolites are similar in their crystalline framework structure, acidity, morphology, and textual properties but considerably different in their catalytic performance for conversion of methanol to olefins (MTO). Such an unexpected but exciting finding was extensively explored by various techniques and density functional theory calculations. A detailed investigation shows that it is the different Al distribution in the ZSM-5 and ZSM-11 framework that causes the significant difference in MTO catalytic performance. In ZSM-5, Al atoms are enriched in the intersection, whereas in ZSM-11, the Al atoms are concentrated in the straight 10-membered ring channel. The acid sites located in the intersection enhance the arene-based cycle that generates more ethene, alkanes, and aromatics. Nevertheless, these hydrocarbon molecules can easily diffuse out of the zeolite channel, hence retarding the deposition of carbonaceous materials and increasing catalytic stability. However, the acid sites located in t...

Understanding the Role of Aluminum in Positive Electrode Material Structures

Application of aluminum substitution (doping) for transition metals are commonly used in lithium ion batteries to improve electrochemical performance, chemical and thermal stability of the cathode materials. However understanding the nature of surface and lattice aluminum environments or understanding possible aluminum diffusion on the interphase have been a challenge for aluminum bearing Li-ion battery materials such as LiNi1-y-zCoyAlzO2 (NCA), aluminum doped LiNixMnyCozO2 (NMC) and coated layered metal oxides. Solid state NMR spectroscopy is one of the few structural probes available that can qualitatively and quantitatively characterize lattice and non-lattice (i.e. surface, coatings, segregation, secondary phase etc.) aluminum coordinations. In the present study, we use NMR to gain new insights into transition metal (TM)-O-Al coordination’s, interfacial composition in pristine cathodes and evolution of lattice aluminum sites and solid electrolyte interface upon cycling. For each cathode composition, aluminum coordination of paramagnetic (lattice) and diamagnetic (non-lattice) nature have been investigated and their effect on cycling performance and surface film composition is reported.

Comparative acidity of BEA and Y zeolite composites with 12-tungstophosphoric and 12-tungstosilicic acids

Catalytic materials supported on solid matrices have driven progress in classical processes in the search for high catalytic activity. This report provides a comprehensive comparison of twelve composites prepared by impregnation of two heteropolyacids (H3PW12O40 and H4SiW12O40) on zeolites BEA and Y (X%HPA/Zeolite). The materials were characterized by XRD, FT-IR, multinuclear MAS NMR, N2 and pyridine adsorption and calorimetry. The Keggin structure of the HPA and zeolite were confirmed by 31P/29Si/27Al MAS NMR, FT-IR and XRD. Generally, for X%HPA/zeolite, dealumination occurred after impregnation. Secondary intrareticular mesopores were more evident on zeolite BEA than on zeolite Y and the decreased specific surface area and pore volume with increased HPA loading was indicative of HPA deposition on their outer surface. There was a relatively strong interaction between the HPA and the zeolite surface, which involved extraframework aluminum (EFAL) sites hydrogen bonded to protons of HPA. 39%HPW/BEA and 40%HPW/Y showed Brønsted sites with high strength (average −145 kJ mol−1), which were strong enough to catalyze the esterification of n-butanol with acetic acid, yielding 83 and 78% conversion, respectively. The catalysts could be reutilized three times without any appreciable loss in the conversion.

Hydrothermal modification of the alumina catalyst for the skeletal isomerization of n-butenes

Hydrothermal modification of the alumina catalyst of n-butenes skeletal isomerization was investigated. It is shown that during the hydrothermal treatment of γ-Al2O3 and subsequent calcination its activity in skeletal isomerization of n-butenes first increases, and then it decreases with a rise of the hydrothermal treatment duration. This behavior is due to a similar change in the content of the strong Lewis acid sites of alumina, which are the active centers of reaction and they can be identified by IR-spectroscopy of the adsorbed pyridine and EPR-spectroscopy of the adsorbed anthraquinone. We report the mechanism of new Lewis acid sites formation at the γ-Al2O3 hydrothermal treatment containing X-ray amorphous component.

Mechanochemistry of the LiBH4–AlCl3 System: Structural Characterization of the Products by Solid-State NMR

The double-cation metal borohydride, Li4Al3(BH4)13, mechanochemically produced from a 13:3 mixture of lithium borohydride (LiBH4) and aluminum chloride (AlCl3), has a low hydrogen desorption temperature; however, the material’s decomposition is accompanied by a large emission of toxic diborane (B2H6). We found that a decrease of the LiBH4:AlCl3 ratio in the starting mixture yields increased amounts of partially chlorinated products that also dehydrogenate at low temperature, but release negligibly small amounts of diborane. Extensive characterization by solid-state NMR spectroscopy (SSNMR) and powder X-ray diffraction (XRD) found that the 11:3 ratio product maintains the Li4Al3(BH4)13-like structure, with additional Cl– anions substituting for [BH4]– compared to the 13:3 mixture. Further decrease of relative LiBH4 concentration in the starting mixture to 9:3 results in a different product composed of tetrahedral [Al(BH4)4]− and [Al(BH4)2Cl2]− complexes, in which two hydrogen atoms of each borohydride group are bridged to aluminum sites. Additionally, SSNMR revealed the covalent character of the Al–H bonds, which is not observed in Li4Al3(BH4)13. These findings suggest that the Al–Cl bonding present in the chlorinated complexes prevents the formation of Al(BH4)3, which is a known intermediate leading to the formation of diborane during thermal dehydrogenation of the nearly chlorine-free Li4Al3(BH4)13.

Steam reforming of acetic acid over Ni/Al2O3 catalysts: Correlation of nickel loading with properties and catalytic behaviors of the catalysts

In this study, steam reforming of acetic acid over the Ni/γ-Al2O3 catalysts with varied nickel contents (from 2.5 to 40 wt%) was performed, aiming to investigate the effects of nickel loading on properties and catalytic behaviors of the catalysts. The results show that the interaction of nickel species with alumina varies with varied nickel loading. At lower nickel loadings, the nickel strongly interacting with alumina are the main species, while with the saturation of the reactive sites of alumina via loading more nickel, the proportion of the nickel species weakly interacting with alumina increased remarkably. Excess nickel on alumina can fill the pores, decreasing the reduction degree of nickel oxides, the specific area (from ca. 277 to 132 m2 g−1 with the increase of nickel loading to 40 wt%), the pore volume, and the pore diameters. Nickel loading also significantly affects the catalytic activity, stability of the catalyst and the resistivity towards coking. With the increase of nickel loading to 10 wt%, the activity of the catalyst reached a plateau. The further increase of nickel loading did not further increase the activity, but increased the stability and the tolerance towards the deactivation induced by coking. In addition, with higher nickel loadings, the coke formed has more large aromatic ring system.

Effect of Ge Substitution on Electronic and Magnetic Properties of Co2Mn(Al1-xGex) Heusler Alloy Using First Principles

In this study we report the effect of Germanium (Ge) substitution at Aluminium (Al) site on the electronic structure of Co2Mn (Al1-xGex) Heusler alloy where x = 0, .25, .75, 1.0 using first principles density functional based method. It has been found that with the increasing Germanium (Ge) content in Co2Mn (Al1-xGex) alloy, lattice constant increases linearly as per Vegard’s law. This increment in lattice constant is due to higher atomic radius of Ge atom. The electronic structure calculation shows that the minority band exhibits a gap around Fermi level confirming the half metallicity of material for all concentrations. We have found that among the other compositions, Co2Mn(Al.75Ge.25) is having higher half metallic character, but quite less magnetic moment whereas Co2MnGe possesses higher magnetic moment but less half metallic character value thereby making the material a good candidate for spintronic devices.

Methanol Oxidation to Formate on ALD-Prepared VOx/θ-Al2O3 Catalysts: A Mechanistic Study

Well-defined supported VOx/θ-Al2O3 catalysts were prepared by atomic layer deposition (ALD) with vanadium coverages of 0.48, 1.20, and 3.40 wt %. In-situ Raman and UV–vis diffuse reflectance spectroscopy confirm that the monovanadate, VO4, is the predominant vanadium species at low loadings (0.48 and 1.20 wt %), while polyvanadate VO4 is the predominant vanadium species for the 3.40 wt % VOx/θ-Al2O3 catalyst. In-situ FTIR spectroscopy of methanol oxidation to formate, in the absence of gas-phase oxygen, on the 0.48 wt % VOx/θ-Al2O3, identified two different formates. A comparison of the frequencies for the formates adsorbed on just V2O5 and on just θ-Al2O3 demonstrates that one of these formates is located on aluminum sites of VOx/θ-Al2O3 while the other is located on vanadium sites. The oxidation state of vanadium for the VOx/θ-Al2O3 catalyst was determined by XPS after different reaction times. On the basis of the time dependence of the formate absorptions and the change in the oxidation state of vanadi...

Hydrogen Adsorption on Nanosized Platinum and Dynamics of Spillover onto Alumina and Titania

An extended theoretical investigation of the electronic and interface properties of titania and alumina with and without supported platinum nanoparticles is presented and compared to recent experimental data with the aim to understand the mechanism of hydrogen activation, adsorption, and spillover. Thirteen-atom platinum particles on titania adopt a distinct different structure than on alumina, which results in distinct hydrogen coverages. Upon hydrogen adsorption, titania is reduced with creation of Ti(III) electronic trap states, strongly interacting with the surface adsorbed protons. The combined Ti(III)/proton migration rate is slower than the one of single surface protons, and it is not influenced by the presence of coadsorbates, such as water molecules. Hydrogen is instead heterolytically split on defect sites on alumina with the formation of a surface proton and a hydride moiety, bound to the particularly reactive surface tricoordinated aluminum site. The electronic structure of alumina is only mar...

The optical properties of Sr3SiAl10O20 and Sr3SiAl10O20:Mn4+

Mn4+-activated luminescent materials have attracted significant attention recently. In particular, alkaline earth aluminates, such as Sr4Al14O25:Mn4+ and CaAl12O19:Mn4+, emit light in the red region, which can be exploited in phosphor-converted LEDs. We applied a sol-gel precursor followed by a ceramic method to synthesize highly crystalline Mn4+-doped Sr3SiAl10O20. The compound Sr3SiAl10O20:Mn4+ exhibits deep red photoluminescence that peaks at 663 nm, which can be assigned to the 2Eg → 4A2g intraconfigurational transition of Mn4+ ([Ar]3d.3 configuration) within the [MnO6]8− octahedra on the aluminum site in the Sr3SiAl10O20 (Space group C12/m1) host structure. The photoluminescence properties, such as the temperature dependence of the luminescence intensity and luminescence lifetime, are presented. Furthermore, the luminescence intensity as function of the activator concentration was investigated. Additionally, the band structure of the undoped host material was treated with Density Functional Theory (DFT). The theoretical results were evaluated experimentally with diffuse UV reflectance spectroscopy. Finally, the crystal field and Racah parameters were extracted to compare with those reported in the literature.