She-Tin Wong

Sulfated zirconia catalyst supported on MCM-41 mesoporous molecular sieve

Sulfated zirconia (SZ) was supported on siliceous hollow tubular MCM-41 mesoporous molecular sieve by using a one-step incipient wetness impregnation method with zirconium sulfate as the precursor. The SZ/MCM-41 catalyst was obtained by thermal decomposition of the precursor in air. The resultant catalyst was characterized with various techniques, such as nitrogen physisorption, X-ray diffraction, SEM, and TEM. It was shown that the well-ordered channels of MCM-41 support arranged in hexagonal arrays while the hollow tubular morphology was retained. Both tetragonal and monoclinic phases of zirconia were developed in the catalysts. With the addition of a proper amount of aluminum as a promoter, resulting in catalyst SZA/MCM-41, the transformation of zirconia from metastable tetragonal phase to monoclinic phase was retarded. The catalytic activity of SZA/MCM-41 catalyst in the isomerization of n-butane was dramatically improved in comparison to the activities of SZ/MCM-41 or SZA/silica.

In-situ study of MCM-41-supported iron oxide catalysts by XANES and EXAFS

Our study focuses on the structural evolution of MCM-41-supported iron oxide under the reducing environment of catalyst pretreatment and ethylbenzene dehydrogenation reaction. Powder X-ray diffraction (XRD) analysis showed that the iron oxide is well-dispersed on the surface of the support with no detectable diffraction peaks from iron oxide. X-ray absorption near edge structure (XANES) study indicates that iron oxide is being reduced during catalyst pretreatment under flowing helium from a-Fe2O3 at room temperature to Fe3O4 at 425C. At 500C, the oxide species is reduced even further. Curve-fitting analysis of the extended X-ray absorption fine structure (EXAFS) radial distribution function (RDF) profile of the catalyst pretreated at 500C can be done with a basic tetragonal g-Fe2O3 spinel structure. However, the cationic vacancies of the spinel on the octahedral position are almost filled with iron cations, indicating that the structure of the iron oxide species is approaching that of a ccp FeO. Stabilization of the FeO-like structure formed at 500C is probably through iron oxide-support interactions, especially via condensation of the oxide terminal hydroxyl groups with the silanols of MCM-41. This distorted form of iron oxide species is metastable and contains labile surface oxide anions, which are probably responsible for the high initial catalytic activity during ethylbenzene dehydrogenation reaction at 500 C. In the presence of the reactant, however, the iron oxide is further reduced and metallic iron is formed during the reaction. The formation of metallic iron is probably through fragmentation of FeO particles, as shown by catalysis and EXAFS results. The reduction process contributes mainly to the deactivation of the catalyst.

Preparation and characterization of MCM-41 and silica supported nickel boride catalysts

Abstract Ni-B/MCM-41 and Ni-B/SiO2 catalysts were prepared by low-temperature reduction of surface-adsorbed nickel chloride with ethanolic sodium borohydride solution. XRD analysis showed that the MCM-41 framework of Ni-B/MCM-41 catalyst remained intact, while nitrogen adsorption–desorption study revealed uniform pore size distribution of the channel system. Thermal treatment of this catalyst under nitrogen at 450°C indicated the presence of a segregated nickel phase on the surface of MCM-41. On amorphous silica, however, amorphous nickel boride alloy may have formed. As-synthesized Ni-B/MCM-41 is more susceptible to oxidation than Ni-B/SiO2 probably due to the smaller size of its alloy particles. This suggestion is also supported by the EXAFS results. The lower activity of Ni-B/MCM-41 catalyst than Ni-B/SiO2 in benzene hydrogenation reaction probably reflects the lower reducibility of nickel oxide phase than nickel boride alloy phase.

Pillared layered manganese oxide

Keggin ion-pillared buserite was prepared by ion-exchanging the hexylammonium ion-expanded buserite with Keggin ions, [Al13O4(OH)24(H2O)12]7+. The starting material was synthetic Na-buserite, which is a layered manganese oxide of composition Na4Mn14O26xH2O. The thermal and redox properties of this oxide and its pillared derivative were compared in O2, N2 and H2 environments using TG, DSC and XRD. The results indicated an improvement in thermal stability of pillared compound relative to Na-buserite in all gaseous environments. By using these compounds in catalysing the oxidation of ethane, it was found that they were very active for complete oxidation.ZusammenfassungKeggin-ionengestütztes Buserit wurde durch Ionenaustausch von hexylammoniumionenerweitertem Buserit mit Keggin-Ionen [Al13O4(OH)24(H2O)12]7+ erhalten. Das Ausgangsmaterial war synthetisches Na-Buserit, ein schichtiges Manganoxid der Zusammensetzung Na4Mn14O26xH2O. In Sauerstoff-, Stickstoff- und Wasserstoffumgebung wurden mittels TG, DSC und Röntgendiffraktion die thermischen und Redoxeigenschaften dieses Oxides und der gestützten Derivate miteinander verglichen. Bei allen gasförmigen Umgebungen erweisen sich die gestützten Verbindungen thermisch stabiler als Na-Buserit. Bei der Anwendung dieser Verbindungen zur Katalyse der Oxidation von Ethan fand man, daß sie bezüglich einer vollständigen Oxidation sehr aktiv sind.

The Synthesis and Characterization of Mesoporous Molecular Sieves MCM-41 with Interconnected Channels

Abstract The mesoporous MCM-41 materials with highly connected nanochannels have been obtained from the aluminosilicate-CnTMAX-H2O systems by a delayed neutralization procedure. The N2 adsorption-desorption isotherm of the materials exhibits a large type B hysteresis at p/p0 0.5. The hysteresis effect is dependent on the synthetic condition and the Si/Al ratio. In spite of the chain length of the surfactant, this hysteresis takes place around the same p/p0 value. From the TEM micrographs of the ultrathin section, one can clearly find that these mesoporous materials contain many structure defects, which are irregular shaped and the size distributes between 5.0-30.0 nm. Combining the above two evidences, we interpret the neighboring channels are interconnected through defects in channels walls. The diffusion of molecules inside these highly defective MCM-41 materials becomes more effective. We show that it leads to a better catalyst support material using ethylbenzene dehydrogenation reaction as an illustration.

W/Zr mixed oxide supported on mesoporous silica as catalyst for n-pentane isomerization

Abstract WO 3 /ZrO 2 mixed oxides supported on various porous silica were prepared. For the mesoporous silica, SBA-15 was found to retain the crystalline structure better than MCM-41 after loading W/Zr mixed oxides. Tungstated zirconia is mainly dispersed into the mesoporous channels of SBA-15, which causes the dramatical decrease in the surface area and pore volume. All the mesoporous siliceous materials supported solid acid samples showed strong acidity. The SBA-15 supported WO 3 /ZrO 2 materials promoted with Pt were highly efficient in catalyzing the isomerization of n-pentane with a high selectivity of iso- pentane. SBA-15 with 1 %Pt/20%WO 3 /40%ZrO 2 gave the highest catalytic activity. This work was supported by China Petroleum Corporation and National Science Council of Taiwan.

Alumina-promoted sulfated mesoporous zirconia and catalytic application in butane isomerization

Mesoporous zirconia (m-ZrO 2 ), possessing high surface area, would be an interesting catalytic material to investigate. Instead of using the conventional strategy to dope S and Al onto m-ZrO 2 separately, our group used aluminium sulfate (abbreviated as AS) as the source of Al and S directly to prepare alumina-promoted sulfated mesoporous zirconia catalysts. The best catalyst in n-butane isomerization reaction was one prepared by the addition of 2 mole% of AS onto m-ZrO 2 with a 630°C calcination temperature. Various characterization techniques were used to locate the surface sites active for the isomerization reaction. The X-ray photoelectron spectra were employed for the identification of acid sites on the catalysts. And the amount of Al loaded onto the catalyst also has a direct influence on the balance of surface acid sites on the catalysts. The best performance of catalysts is optimized at a W T value of ∼ 4.4 wt% and S/A1 value of ∼ 1.7.

Molecular sieves from pillaring of layered silicates

Silica- and alumina-pillared derivatives of a family of layered silicates with various basal layer charge density and thickness were prepared. The hexylamine-expanded silicates were used as the precursors for pillaring reactions either in aqueous or non-aqueous media. Silicapillared derivatives were prepared by dissolving tetraethylorthosilicate into the organophilic interlayer region, while alumina-pillared derivatives were prepared by ion-exchange of the interlayer hexylammonium ions with aluminum Keggin ions in aqueous solution. The characterization techniques used in this study include XRD, MAS-NMR and N, adsorptiondesorption isotherms. Microporosity was obtained in all the pillared derivatives, while the alumina-pillared assemblies had relatively lower surface areas. The layer thickness of the silicates showed direct correlation with the thermal stability of the pillared products.