Oxidative activation of n-butane on sulfated zirconia

Catalytic activation and conversion of light alkanes by sulfated zirconia is unequivocally shown to be initiated by producing small concentrations of olefins. This occurs via stoichiometric oxidative dehydrogenation of butane by SO3 or pyrosulfate groups to butene (present mostly as alkoxy groups), water, and SO2. Thermal desorption and in situ IR spectroscopy have been used to determine all three reaction products. The concentration of butene formed determines both the catalytic activity of sulfated zirconia as well as the deactivation via formation of oligomers. The thermodynamics of the oxidative dehydrogenation of n-butane by different SZ surface structures has been examined by density functional (DFT) calculations. The calculations show that pyrosulfate or re-adsorbed SO3 species have the highest oxidizing ability.     

Studies on platinum-promoted sulfated zirconia alumina: effects of pretreatment environment and carrier gas on n-butane isomerization and benzene alkylation activities

A series of platinum-promoted sulfated zirconia alumina catalysts (SZA) with different amounts of platinum (0.5, 1, and 2 wt%) were synthesized. Two other catalysts were prepared by mechanically mixing different proportions of the Al-promoted sulfated zirconia with Pt/Al(2)O(3). The 650 degrees C calcined catalysts were characterized by N(2) adsorption/desorption (BET), TPR, and TPD analysis. Butane isomerization activity of the catalysts was studied at 270 degrees C, varying the pretreatment environment and carrier gases. Though the textural properties of the catalysts did not change significantly with platinum loading, the maximum surface area of 116 m(2)/g was exhibited by the catalyst with 1 wt% Pt loading. Under the studied reaction conditions, the air-pretreated catalysts (sulfated zirconia alumina (SZA) and platinated SZA) showed higher n-butane conversion than the N(2)-pretreated catalyst. However, nitrogen was a better carrier gas than H(2), CO(2) or air, and CO(2) and air deactivated the catalyst very fast. Unlike the platinated SZA catalysts, the mechanically mixed catalysts showed an induction phenomenon. A redox mechanism is suggested for butane isomerization over these catalysts. The catalyst SZA was also found to be active for alkylation of benzene with isopropanol, which gave 93% selectivity toward cumene.     

Effect of metals on the catalytic activity of sulfated zirconia for light naphtha isomerization

We studied on the function of the metal in the sulfated zirconia(SO₄^2–/ZrO₂) catalyst for the isomerization reaction of light paraffins. The addition of Pt to the SO₄^2–/ZrO₂ carrier could keep the high catalytic activity. The improvement in this isomerization activity is because Pt promotes removal of the coke precursor deposited on the catalyst surface. Though this catalytic function was observed in other transition metals, such as Pd, Ru, Ni, Rh and W, Pt exhibited the highest effect among them. It was further found that the Pd/SO₄^2–/ZrO₂–Al₂O₃ catalyst possessed a catalytic function for desulfurization of sulfur-containing light naphtha in addition to the skeletal isomerization. The sulfur tolerance of catalyst depended on the method of adding Pd, and the catalyst prepared by impregnation of the SO₄^2–/ZrO₂–Al₂O₃ with an aqueous solution of Pd exhibited the highest sulfur tolerance.Further, we investigated the improvement in sulfur tolerance of the Pt/SO₄^2–/ZrO₂–Al₂O₃ catalyst by impregnation of Pd. The results of EPMA analysis indicated that this catalyst was a hybrid-type one (Pt/SO₄^2–/ZrO₂–Pd/Al₂O₃) in which Pt/SO₄^2–/ZrO₂ particles and Pd/Al₂O₃ particles adjoined closely. This hybrid catalyst possessed a very high sulfur tolerance to the raw light naphtha that was obtained from the atmospheric distillation apparatus, although this light naphtha contained much sulfur. We assume that such a high sulfur tolerance in the hybrid catalyst is brought about by the isomerization function of Pt/SO₄^2–/ZrO₂ particles and the hydrodesulfurization function of Pd/Al₂O₃ particles. Besides, since the hybrid catalyst also provides high catalytic activity in the isomerization of HDS light naphtha, we suggest that the Pd/Al₂O₃ particles supply atomic hydrogen to the Pt/SO₄^2–/ZrO₂ particles by homolytic dissociation of gaseous hydrogen and also enhance the sulfur tolerance of Pt/SO₄^2–/ZrO₂ particles. Finally, we also propose the most suitable location of Pd and Pt in the metal-supported SO₄^2–/ZrO₂–Al₂O₃ catalyst.     

Comparison of the promoting effects of gallium and aluminium on the n-butane isomerization actvity of sulfated zirconia

Sulfated zirconia catalysts promoted by gallium or aluminium were prepared and tested for n-butane isomerization. Both elements increase and stabilize the isomerization rate. Characterization results showed that the promoting mechanism of gallium was different from that of aluminium. This revised version was published online in June 2006 with corrections to the Cover Date.     

Influence of platinum and hydrogen on n-butane isomerization on H-Mordenite

Hydrogen inhibits n-butane isomerization of H-MOR and improves catalyst stability and selectivity. Similarly, when Pt is present, hydrogen inhibits the reaction which is always controlled by the acid function.     

Effect of Mn and Fe on the reactivity of sulfated zirconia toward H2 and n-butane: a diffuse reflectance IR spectroscopic investigation

Sulfated zirconia (SZ) and sulfated zirconia promoted with 2 wt % manganese (MnSZ) or iron (FeSZ), all active in n-butane isomerization, were investigated using diffuse reflectance Fourier transform IR spectroscopy (DRIFTS). By adsorption of H(2) at 77 K or of n-butane at room temperature, it was found that the promoters neither enhance the Lewis nor the Br?nsted acid strength. SZ and promoted SZ do not exhibit higher acid strength than zeolites. In a batch experiment using 70 hPa of H(2), SZ did not react at 473 K. Reaction of H(2) with MnSZ produced water (band at 5242 cm(-1)) and a decrease in the sulfate groups (multiple bands). Heating of SZ in 10 hPa n-butane to 573 K caused total reduction of sulfate to H(2)S (2583, 2570 cm(-1)) and partial and total oxidation of butane to olefinic species (3062 cm(-1)), CO(2), and water. MnSZ and FeSZ reacted with n-butane already at 373 K; products of skeletal isomerization (methyne CH vibration at 2910 cm(-1)) were detected and sulfate groups were consumed. Rather than increasing the acidity, the promoters enhance the oxidation potential of sulfate and facilitate alkane activation via oxidative dehydrogenation.     

Effect of solvation and confinement on the trans-gauche isomerization reaction in n-butane

The effect of solvation and confinement on the conformational equilibria and kinetics of n-butane is examined using molecular dynamics simulations of the bulk and confined fluids and compared to appropriately chosen reference states. Clear evidence for a solvent shift of the preferred conformation in bulk n-butane is found. At a temperature of 292 K and a density of 6.05 nm-3 a small solvent shift in favor of gauche is observed (similar to previously reported values), and the shift increases substantially with an increase in density to 8.28 nm-3. The rate of torsional interconversion from the trans to the gauche state, calculated using the relaxation function method, was found to increase with increasing temperature and density. The rate constants kTG and kGT have an Arrhenius temperature dependence yielding activation energies significantly lower than the trans-gauche and gauche-trans barrier heights in the torsional potential for a free molecule, depending on the density. In the confined phase, we considered the same densities as simulated in the bulk phase, and for four different values of the physical pore width (approximately 1.5-4.0 nm). At the high density, we find that the position of the trans-gauche equilibrium is displaced towards excess trans compared with the bulk phase, reflecting the confinement and interactions of the molecules with the pore wall. The isomerization rate is found to decrease with decreasing pore width. Again, we find that the kinetics obeys an Arrhenius rate law and the activation energy for the trans-gauche and gauche-trans interconversions is slightly smaller than that of the bulk fluid at the same density.     

Superoxide radical anions on the surface of zirconia and sulfated zirconia: formation mechanisms, properties and structure

In situ ESR spectroscopy has been used for direct comparison of different thermal and light-induced processes leading to generation of superoxide radical anions on the surface of various zirconia and sulfated zirconia materials. For materials of both types the magnetic resonance parameters of the radical anions were found to be practically independent of the generation method, except for oxygen coadsorption with NO that yields radicals with somewhat smaller gz values. The parameters appear to depend mostly on the state of the surface zirconia cations stabilizing the radical anions, so that the g tensor anisotropy is significantly smaller over sulfated zirconia. It is shown that light-induced formation of superoxide radical anions in the presence of coadsorbed hydrocarbons can be initiated with visible light. Original SIET reaction mechanisms are suggested for the formation of superoxide radical anions by coadsorption with hydrocarbons and illumination after such coadsorption to extend the previously known ones to account for the observed phenomena. Cluster model DFT calculations of magnetic resonance parameters of O2- radical anions stabilized on the surface of zirconium dioxide showed that the adsorption complexes have a -shape rather than linear structure. The magnetic resonance parameters obtained by calculations practically match experimental data and adequately describe their changes after the surface modification with sulfates.     

The effects of ammonium sulfate and sulfamic acid on the surface acidity of sulfated zirconia

In this work, sulfated zirconia (SZ) was prepared by a solvent-free method, ammonium sulfate ((NH₄)₂SO₄) and sulfamic acid (NH₂SO₃H) were as sulfur sources, respectively. The resulting catalysts were characterized by powder X-ray diffraction (XRD), thermogravimetric analysis and differential scanning calorimetry (TGA-DSC), N₂ adsorption–desorption isotherms, inductively coupled plasma (ICP) and pyridine-FTIR. The characterization of the catalysts revealed that SZ prepared with (NH₄)₂SO₄ as sulfur source contained more Lewis acid sites and less Brønsted acid sites. The effects of sulfur sources on the catalytic activity of catalysts were studied. Catalytic testing showed that SZ prepared with (NH₄)₂SO₄ as sulfur source had a better catalytic performance for removing trace olefins from aromatics.     

Comparative study of regioselective synthesis of beta-aminoalcohols under solventless conditions catalyzed by sulfated zirconia and SZ/MCM-41

Sulfated zirconia and SZ/MCM-41 were used as catalysts for the synthesis of beta-aminoalcohols via epoxide aminolysis. Sulfated zirconia was prepared by sol-gel and SZ/MCM-41 was obtained by impregnation. Solid catalysts were characterized by XRD, SEM-EDS, UV-Vis, FT-IR pyridine desorption and Nitrogen physisorption. Both acid materials were useful as catalysts, even when they were recycled several times. The beta-aminoalcohols were characterized by FT-IR, (1)H- and (13)C-NMR and GC-MS.     

Multicomponent synthesis of 4-aryl-NH-1,2,3-triazoles in the presence of Al-MCM-41 and sulfated zirconia

The acid properties of Al-MCM-41 and sulfated zirconia were used to develop a strategy for the synthesis of 4-aryl-NH-1,2,3-triazoles via a multicomponent reaction involving various benzaldehydes, sodium azide and nitromethane. The efficiency of the process is analysed from the perspective of the difference in acids among the catalysts used.     

Density functional theory study of the reactivity and electronic structure of the transesterification of triacetin in biodiesel production via a sulfated zirconia heterogeneous catalysis

This DFT study examined the interaction of a sulfated zirconia (SZ) slab model system (heterogeneous catalyst) and triacetin (a precursor in biodiesel production) using explicit methanol solvent molecules. Full geometry optimizations of the systems were performed at the B3LYP level of theory. Gibbs free energies provide insight into the spontaneity of the reactions along a three‐step reaction mechanism for the transesterification of triacetin. Charge decomposition analysis revealed electronic charge transfer between the metallic oxide and the organic moieties involved in the reaction mechanism. Fukui indices indicate the likely locations on the SZ surface where catalysis may occur. The quadratic synchronous transit scheme was used to locate transition structures for each step of the transesterification process. The results are in agreement with the strongly acidic catalytic character of zirconium observed experimentally in the production of biodiesel. © 2016 Wiley Periodicals, Inc.     

Theoretical study on the isomerization and tautomerism in barbituric acid

Isomerization and tautomerism of 16 isomers of barbituric acid (BA) were studied at the MP2 and B3LYP levels of theory. Activation energies (E _a), imaginary frequencies (υ), and Gibbs free energies (ΔG ^#) of the amine-imine and keto-enol tautomerisms and O–H internal rotations were calculated. The activation energies of amine-imine tautomerisms were in the range of 110–200 kJ/mol and for keto-enol tautomerisms were larger than 200 kJ/mol. The calculated activation energies of internal O–H rotations were smaller than 60 kJ/mol. Effect of micro-hydration on the transition state structures and activation energies of the tautomerisms were also investigated. Water molecule catalyzed the tautomerisms and decreased the activation energies of both the amine-imine and keto-enol tautomerisms about 100–120 kJ/mol.     

Quantum chemical study on isomerization and transformation of hexabromocyclododecanes

Structural Chemistry - Hexabromocyclododecanes (HBCDs) have been frequently detected in environment. The isomerization between HBCDs and thermodynamic properties of unimolecular and bimolecular...     

PALS study on the defect structure of yttria-stabilized zirconia

Powders of stabilized ZrO₂–8 mol% Y₂O₃ (YSZ) have been obtained by mechanical milling in zirconia vials. The samples were characterized by X-ray diffraction (XRD). Positron annihilation lifetime (PALS) measurements were performed to investigate the lattice defects originated by the incorporation of yttria and those mechanically induced. The XRD results indicate the formation of tetragonal YSZ solid solution. PALS results indicate that positron trapping occurs at different kinds of defects such as vacancy-like defects, grain boundary and associated defects.     

First-principles study of the structure,mechanical properties,and phase stability of crystalline zirconia under high pressure

We presented a detailed study on the structure, mechanical properties, and phase stability for three zirconia crystals under hydrostatic pressure of 0–100 GPa by using density functional theory within the generalized gradient approximation. It is found that m-ZrO₂ presents three phase transitions with increasing pressure, while t-ZrO₂ and c-ZrO₂ do not. As the pressure increases, the band gap of m-ZrO₂ presents three abrupt changes. The band gap of t-ZrO₂ firstly decreases and then increases slowly. The band gap of c-ZrO₂ increases monotonically. An analysis of elastic constants shows that the three oxides are anisotropic under compression with increasing pressure. As the pressure increases, their fracture strength and plastic strength are improved and they are all ductile. The calculated formation enthalpies suggest that the elements (Zr + O₂) are able to form m-ZrO₂, t-ZrO₂, or c-ZrO₂ in the whole pressure range, indicating that zirconium and its alloys are easy to be oxidized.