Modification of the catalytic properties of sulfated zirconia by addition of metal promoters

We have studied the effects of promoting sulfated zirconia catalysts by addition of transition metals. We have found that the addition of Ni causes an activity enhancement comparable to that caused by the addition of Fe and Mn. The TPD/DRIFTS results show that the catalytic activity does not correlate with either Brønsted or Lewis acidity. The high isomerization activity of these catalysts is explained in terms of a bifunctional mechanism in which the metal promoters are responsible for an enhancement in the surface concentration of olefins rather than an enhanced acidity.     

Acoustic characterization and microstructure of high zirconia electrofused refractories

A new line of electrofused refractory materials with a very high content of zirconia (HZ) has been developed to satisfy the needs of new generation manufacturing glass furnaces. Such materials are subjected to severe operating conditions (temperature and corrosion) during their manufacturing and service life. These HZ materials required very high temperature casting and a suitable annealing process to prevent defects and cracks during manufacturing. Therefore, a research program has been launched to build numerical tools able to predict the thermo-mechanical behaviour of these materials during one of the most critical phases of manufacturing: the controlled cooling of the refractory blocks after melting and casting. The efficient development of such a tool requires the knowledge of thermo-mechanical properties of these materials with temperature, in conditions close to that occurring during processing.In the framework of this approach, the present paper deals with the characterization of elastic properties of two HZ materials, using two mechanical testing devices i.e. a pulse echography technique and a tensile test device. An innovative acoustic emission device is also used to help in identification of microdamage occurrence. The goal of this study is to investigate the microstructure organisation of materials at very fine scale (<100 μm) in order to correlate the obtained results with the macroscopic properties of the material. Characterisations are also performed at intermediate temperature to establish correlations with the manufacturing process.     

Study of the preasphaltenes of coal liquefaction and its hydro-conversion kinetics catalyzed by SO4/ZrO2

The investigation of hydro-conversion behavior of the heavy intermediate products derived from coal direct liquefaction is advantageous to optimize the technological conditions of direct coal liquefaction and improve the oil yield. In this paper, the hydro-conversion of preasphaltenes catalyzed by SO₄²⁻/ZrO₂ solid acid was investigated based on the structural characterization of preasphaltenes and its hydro-conversion products, and the determination of products distribution and the kinetics of preasphaltenes hydro-conversion. The results indicated that the content of condensed aromatic rings increased, and the contents of hydrogen, oxygen and aliphatic side chains of preasphaltenes decreased with the increase of coal liquefaction temperature. The preasphaltenes showed higher hydro-conversion reactivity while SO₄²⁻/ZrO₂ solid acid was used as catalyst. Higher temperature and longer time were in favor of increasing the conversion and the oil + gas yield. The conversion of preasphaltenes hydro-conversion under 425 °C, for 40 min reached 81.3% with 51.2% oil + gas yield. SO₄²⁻/ZrO₂ solid acid was in favor of the catalytic cracking rather than the catalytic hydrogenation in the hydro-conversion of preasphaltenes. The activation energy of preasphaltenes conversion into asphaltenes was 72 kJ/mol. The regressive reactions were only observed at a higher temperature.     

Preparation and characterization of zeolite catalysts for etherification reaction

Catalytic etherification of 2-naphthol with ethanol has been carried out over a series of solid-acid catalysts such as H-Beta, H-MOR and H-ZSM-5 using a flow-type reactor. H-Beta zeolite shows higher conversion and catalytic stability than other catalysts for the production of 2-naphthyl ethyl ether, which may be correlated to the amount and strength of acid sites. H-Beta zeolites with different Si/Al ratios show that conversion decreases with increasing Si/Al ratios. The NH₃-TPD profiles indicate that the weak acidity decreases more sharply with increase in Si/Al ratios, compared with the strong acidity. The influences of ethanol/2-naphthol molar ratios, reaction temperature, and space velocity with respect to catalytic activities are investigated for H-Beta zeolite in the present work.     

Preparation and characterization of multicomponent porous materials prepared by the sol-gel process

An experimental strategy was developed to obtain transparent Si-Al-Ti-Ni-Mo and Si-Zr-Ti-Ni-Mo sols via the sol-gel process. The sol was prepared from Si(OEt)₄ (TEOS), Al(OBu^s)₃ (OBu^s: C₂H₅CH(CH₃)O), Ti(OEt)₄ (OEt: OCH₂CH₃), Zr(OPrⁿ)₄ (OPrⁿ: OCH₂CH₂CH₃). In both cases nickel nitrate hexahydrate (Ni(NO₃)₂ · 6H₂O) and ammonium heptamolybdate tetrahydrate ((NH₄)₆Mo₇O₂₄ · 4H₂O) were the Ni and Mo sources, respectively. The sols were characterized by Fourier Transform Infrared Spectroscopy (FTIR). Assignments of the simultaneous formation of the Si-O-Al, Si-O-Ti, Si-O-Ni, and Si-O-Zr bonds were done. The sols were polymerized at room temperature (293 K) to obtain gels, and these were dried at 423 K and calcined at 573, 853 and 893 K in air. The characterization techniques used were small-angle X-ray scattering (SAXS), X-ray diffraction (XRD), scanning electron microscopy (SEM), and ²⁹Si and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR). The density of the solids was measured following ASTM method D-4892 and the porosity and surface area were determined by N₂ adsorption/desorption isotherms. The corresponding average pore diameters were evaluated using the BJH, HK, and DA methods.     

Microorganism adhesion inhibited by silver doped Yttria-stabilized zirconia ceramics

The aim of this study was to incorporate silver nanoparticles into yttria-stabilized zirconia (YSZ) ceramic for eliminating microorganism adhesion on dental restoration graft. Y₂O₃ (3% mol) partially stabilized ZrO₂ powder with particle size around 80 nm was employed to fabricate tetragonal phase dominated YSZ ceramic block. Silver nanoparticles were efficiently loaded at pH 9.5 by NaBH₄ reducing of AgNO₃. The biocompatibility of silver incorporated YSZ was evaluated by MTT assay. Antimicrobial activities were quantitatively determined by optical density measurement and qualitatively analysed by scanning electro microscope. Inductively coupled plasma-mass spectrometry (ICP-MS) revealed that YSZ block containing 0.0047 wt% nanosilver, which is safe to mammalian cell, can inhibit the growth of Escherichia coli, Streptococcus mitis and Candida albicans. The pristine YSZ disc had no effect on bacterial growth. This study suggests that silver nanoparticles incorporated into YSZ blocks possess a broad spectrum antimicrobial effect and may help prevent biofilm formation on their surfaces to improve implant survival rate.     

CSL grain boundary distribution in alumina and zirconia ceramics

The distributions of general and coincidence site lattice (CSL) grain boundaries (GBs) in texture-free alumina and zirconia ceramics sintered at two different temperatures were investigated based on electron backscatter diffraction (EBSD) measurements. Results were compared with the distributions obtained from random 2D spatial models and with calculated random distributions reported in the literature. All alumina samples independent on sintering temperature show the same characteristic deviations of the measured general GB distributions from the random model. No such features can be seen in zirconia. The total fractions of CSL GBs in alumina and zirconia samples are clearly larger, for both sintering temperatures, than those observed in the random simulations. A general GB prominence factor, similar to the twin prominence factor for fcc metals, was defined to simplify the representation of the CSL GB content in zirconia. The observed deviations from the random model show no dependence on sintering temperature nor on lattice geometry. In alumina, however, the change in the CSL GB character distribution with sintering temperature seems to be crystallographically controlled, i.e. directly dependent on the orientation of the CSL misorientation axis.     

Processing of alumina-coated tetragonal zirconia materials and their response to sliding wear

Alumina-coated tetragonal zirconia stabilised with 3 mol% of Y₂O₃ (YTZP) specimens (30 mm × 30 mm × 6 mm) have been obtained by dipping of pre-sintered YTZP compacts in alumina suspensions and subsequent sintering. The coated specimens present hardness values and a wear resistance similar to those of reference dense alumina specimens and significantly higher than those of the YTZP substrates.     

Infrared and raman characterization of V<Subscript>2</Subscript>O<Subscript>5</Subscript> on zirconia modified with WO<Subscript>3</Subscript> and activity for acid catalysis

Vanadium oxide supported on zirconia modified with WO₃ was prepared by adding Zr(OH)₄ powder into a mixed aqueous solution of ammonium metavanadate and ammonium metatungstate followed by drying and calcining at high temperatures. The characterization of prepared catalysts was performed by using FTIR, Raman, and XRD. In the case of calcination temperature at 773 K, for samples containing low loading V₂O₅ below 18 wt%, vanadium oxide was in a highly dispersed state, while for samples containing high loading V₂O₅ equal to or above 18 wt%, vanadium oxide was well crystallized due to the high V₂O₅ loading on the surface of ZrO₂. The ZrV₂O₇ compound was formed through the reaction of V₂O₅ and ZrO₂ at 873 K, and the compound decomposed into V₂O₅ and ZrO₂ at 1,073 K, these results were confirmed by FTIR and XRD. Catalytic tests for 2-propanol dehydration and cumene dealkylation have shown that the addition of WO₃ to V₂O₅/ZrO₂ enhanced both catalytic activity and acidity of V₂O₅-WO₃/ZrO₂ catalysts. The variations in catalytic activities for both reactions are roughly correlated with the changes of acidity.     

Effect of the volume ratio of zirconia and alumina on the mechanical properties of fibrous zirconia/alumina bi-phase composites prepared by co-extrusion

Fibrous zirconia/alumina composites with different composition were fabricated by piston co-extrusion. After a 3rd extrusion step and sintering at 1600 °C, crack-free composites with a fibre width of 50 μm were obtained for all compositions. The effect of the volume ratio of secondary phase on the mechanical properties was investigated. The Young's modulus of the composites decreased linearly with increasing the zirconia content. The fracture toughness of the composites was improved by introducing fine second phase filaments into the matrix. The maximum fracture toughness of 6.2 MPa m^1/2 was attained in the 3rd co-extruded 47/53 vol% zirconia/alumina composite. The improvement in toughness was attributed to both “stress-induced” transformation of zirconia and a crack deflection mechanism due to thermal expansion mismatch between the two phases. Bending strength of the composites was almost the same as that of the monolithic alumina regardless of the composition.     

Onset coarsening-coalescence kinetics of ZrO2 and less refractory TiO2 nanoparticles: Effect of homologous temperature and phase transformation

Specific surface area change of ZrO₂ (predominant tetragonal - (t) symmetry, 30-50 nm) and less refractory TiO₂ anatase nanoparticles (20-50 nm) upon isothermal firing at 700-1000 °C in air was determined by N₂ adsorption-desorption hysteresis isotherm. The nanoparticles underwent onset coarsening-coalescence within minutes without appreciable phase transformation for TiO₂, but with extensive transformation into monoclinic (m-) symmetry for ZrO₂. The apparent activation energy of such a process being not much higher for ZrO₂ (77 ± 23 kJ/mol) than TiO₂ (56 ± 3 kJ/mol) nanoparticles can be attributed to transformation plasticity. The minimum temperature for coarsening/coalescence of the present ZrO₂ and TiO₂ nanoparticles was estimated as 710 and 641 °C, respectively.     

Fabrication of bulk nanocrystalline alumina–zirconia materials

Eutectic mixtures of alumina and zirconia have been successfully transformed to thick amorphous coatings and free-standing parts by plasma spraying. The as-sprayed material has very low open porosity and crystallizes when heated above 950 °C. Short annealing times at the crystallization temperature result in the formation of a very fine microstructure throughout the sample volume with average grain size below 15 nm. The fine nanocomposite structure was confirmed by transmission electron microscopy. High hardness and abrasion resistance of the as-sprayed materials are significantly improved upon annealing due to the nanocomposite structure, which makes the bulk nanocrystalline ceramic material attractive.     

NH3-TPD and FTIR spectroscopy of pyridine adsorption studies for characterization of Ag- and Cu-exchanged X zeolites

Pure crystalline NaX zeolites with Si/Al = 1.2 is prepared by the hydrothermal method. The modification is achieved by following the conventional ion-exchange technique to obtain Ag- and Cu-exchanged forms of NaX zeolite with different metal loadings ranging from low to high exchange. Compositional and structural investigations of all samples are performed by atomic absorption spectrometry, elemental analysis, powder XRD, and nitrogen adsorption porosimetry. The acidity of all exchanged zeolites is investigated using both the ammonia temperature-programmed desorption (NH₃-TPD) carried out by microbalance, and the pyridine adsorption performed via Fourier transform infrared spectroscopy (FTIR). XRD analysis shows the global conservation of the zeolitic structure after ionic exchange. The crystallinity loss is more important for zeolites exchanged at high degree of copper. Porosity measurements indicate a decrease of the porous volumes and the specific surface areas when the Ag or Cu exchange degree is raised. Mesopores particularly appear at high exchange degree of copper. NH₃-TPD indicates that the increase of Ag and Cu-exchange degrees leads to an increase of the global acidity, which is more pronounced for Cu-exchanged zeolites. The strength of acid sites for Ag-exchanged zeolites is weaker than for Cu-exchanged zeolites. The FTIR spectroscopy analysis by pyridine adsorption confirms the formation of extra framework aluminum (EFAL) species, inducing an increase in the concentration and the strength of the Lewis acidity. The formation of EFAL is more important for Cu-exchanged zeolites.     

Synthesis of bulk tetragonal zirconia without stabilizer: The role of precursor nanopowders

The densification of undoped zirconia nanopowder was performed using the High-Pressure Field Assisted Sintering technique, with the aim of preparing bulk size-stabilized tetragonal zirconia. The role played by the structural and microstructural properties of the starting powders on the characteristics of the sintered materials was investigated by exploring several different synthetic routes. Nanopowders prepared by solvothermal methods were proven to give the most satisfactory results thanks to their microstructure, particularly characterized by uniform spherical microaggregates. Using solvothermally synthesized powder as a starting material, nearly fully dense (97% relative density) samples of undoped size-stabilized tetragonal zirconia were obtained with a sintering cycle of 5 min under a uniaxial pressure of 700 MPa, at sintering temperatures as low as 900 °C.     

First-principles DFT study of SO2 and SO3 adsorption on 2PANI: a model for polyaniline response

Interaction of SO_x (x?=?2,3) molecules on active sites of dianiline (as a model for polyaniline, denoted here as 2PANI) was studied using density functional theory at the BLYP-D/6-31+G(d) level of theory. Natural population analysis was used to find out the charge distribution as well as the net transferred charge of SO_x upon adsorption on 2PANI and the result has been compared with Mulliken charge analysis to evaluate the sensing ability of 2PANI. The computed density of states point to the remarkable orbital hybridization between SO_x and 2PANI during the adsorption process. As a consequence, the results of UV–VIS confirm the sensing ability of 2PANI toward SO₂ and SO₃. Based on our results, it can be found that at proper configuration the SO₂ and SO₃ molecules can be adsorbed on 2PANI with adsorption energies (E_ads) of ?18.2 and ?62.9?kJ/mol (BSSE), respectively.     

Microwave-assisted epoxidation of styrene with molecular O2 over sulfated Co–Y–ZrO2 solid catalyst

The microwave-assisted styrene epoxidation reaction with molecular O₂ as an oxidant was studied over a sulfated Co–Y-doped ZrO₂ solid catalyst. The microwave irradiation (400 W) resulted in similar styrene conversion and styrene oxide selectivity, in reduced time, as compared to conventional thermal heating. Higher power (800 W) of microwave irradiation decreased the styrene oxide selectivity as well as leading to the formation of styrene glycol. DMF was found to be the most suitable solvent for epoxidation of styrene with molecular O₂ under microwave irradiation and yielded maximum oxide selectivity (91%) at 120 °C. The microwave-assisted oxidation reaction resulted in time saving and is energy conserving method.     

Adsorbed species of methanol on zirconia support and molybdenum oxide monolayer. Its role in the methanol oxidation

Methanol was strongly adsorbed on ZrO₂ support as dioxymethylene and carbonate species, both of which were adsorbed on the basic sites as the intermediate for complete oxidation. On the molybdenum oxide monolayer, it was adsorbed as methoxide species to form formaldehyde. Upon elevating the temperature, however, the methoxide species turned into formate ion, from which CO was finally desorbed. Adsorption of methoxide species readily takes place only on the loaded molybdenum oxide monolayer, and the acid sites seem to accommodate the species.     

Synthesis of superacidic mesoporous alumina and its application in the dehydration of glycerol

Superacid ZrO₂/SO₄²⁻ (SZ) was supported on mesoporous alumina (MA) to synthesize a novel superacid catalyst (SZMAP) with high active surface area. The synthesized catalyst was characterized by both physical and chemical methods, including XRD, BET, NH₃-TPD and TEM. The prepared superacidic mesoporous alumina (SZMAP) showed typical gamma-alumina phase patterns and uniform mesoporous structures with a regular channel arrangement. NH₃-TPD analysis indicated that SZMAP has superacidic characteristics. The SZMAP catalyst was used in the dehydration of glycerol to acrolein. Results showed that SZMAP showed much higher catalytic activity than the bulk superacid, ZrO₂/SO₄²⁻ (SZ), and MA due to the high dispersion of active SZ molecules, which results from a large surface area and uniform mesopores of mesoporous alumina.     

水热一步法合成SO42-/Zr-PHTS及其催化性能

在低酸度硫酸体系中,以P123(EO₂₀PO₇₀EO₂₀)为模板剂水热法一步合成硫酸化锆掺杂PHTS固体酸催化剂(SO₄^2-/Zr-PHTS),利用XRD、TEM、N₂吸附-脱附以及NH₃-TPD、Py-FTIR等手段对其进行表征。结果表明,SO₄^2-/Zr-PHTS具有有序的六方相介孔结构,具有以L酸为主的弱、中强度的酸性中心;随着硅锆摩尔比的增加,其酸量逐渐减少,比表面积逐渐增加,孔容、孔径无显著变化。SO₄^2-/Zr-PHTS在催化四氢呋喃聚合中表现出良好的反应性能,推测固体酸的表面酸性和孔结构决定着催化四氢呋喃聚合的反应性能。