Vu Anh Tuan

Disruption of the MFI framework by the incorporation of gallium

The influence of isomorphous substitution of silicon by gallium in the MFI framework on lattice crystallinity and defect sites is investigated. The Ga-ZSM-5 zeolites are characterized by XRD, n-hexane adsorption, IR, TPD of ammonia, TG–DTA, MAS NMR, and chemical analysis. The incorporation of the large gallium atom is found to be disruptive. It leads to greater lattice distortion. However, the number of lattice defects decreases with growing gallium content. The sum of lattice defects and incorporated gallium atoms is nearly unchanged. It is suggested that gallium atoms and defect sites are located at similar lattice positions.

Acid properties of ZSM-20-type zeolite

The acid properties of an HZSM-20 Zeolite have been characterized, in comparison to an Hy Zeolite, by IR spectroscopy, temperature-programmed desorption (TPD) of ammonia, time-resolved temperature-programmed FTIR of ammonia desorption and by microcalorimetry. Also, catalytic properties were tested for n-hexane conversion. These studies show that for HZSM-20 there is a shift of the bridging OH band to lower wavenumbers, an increase in the ammounia desorption temperature and a higher initial heat of ammonia chemisorption compared to HY. Thus, HZSM-20 contains medium–strong Bronsted sites, ranging in strength between zeilites HZSM-5 and HY. Obviously, the higher Si: Al ratio of HZSM-20 results in an enhanced number of strong acid sites with no second-neighbour aluminium atoms. This leads to a change in the distribution of Bronsted-acid sites. Superacid Bronsted sites could not be detected. Furthermore, the IR spectra of adsorbed pyridine and ammonia revealed the presence of at least two kinds of Lewis-acid sites. Some Lewis sites of HZSM-20 exhibit a weak acid strength and the strong lewis sites are weaker than those of HY. The different acidic properties of both zeolites are reflected in their catalytic activity and selectivity in n-hexane conversion.

X-ray absorption spectroscopic study on the structure and crystallization of Ga-containing MFI-type zeolites

Abstract The local environment of the Ga atoms in a series of gallosilicates with MFI structure as well as in the dried synthesis gel has been investigated by GaK XANES and EXAFS. In fully crystallized gallosilicate MFI samples, Ga is tetrahedrally coordinated by oxygen atoms with a GaO bond distance of 1.83 A. Lack of order beyond the first coordination shell is in agreement with Ga located in several different crystallographic sites of the zeolite-type framework. For the gel, the same oxygen coordination is observed for the first coordination shell. As expected for the amorphous gel, no order beyond the first coordination shell is detected. Thus, X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) cannot differentiate between tetrahedrally coordinated Ga atoms in a gel phase and Ga atoms incorporated into the framework. Inspection of a series of samples prepared with different crystallization times and with Si Ga ratios>40 show that during crystallization framework and extra-framework Ga remain in tetrahedral coordination. A sample with a low Si Ga ratio of 19 in the gel, a value far below the so far observed maximum Ga framework content of Si Ga ≈40 , contains also octahedral Ga after prolonged hydrothermal treatment. A quantitative evaluation shows that extra-framework Ga in gallosilicate MFI samples occurs in both tetrahedral and octahedral coordination. This result on the nature of extra-framework Ga is of importance, since these Ga species are believed to play a decisive role in catalytic processes on gallosilicate MFI-type solids.

Study on the Isomorphous Substitution of Silicon by Tetravalent Elements (Zr, Ge, Ti) in the Framework of MFI Type Zeolites

Abstract Silicalite samples containing tetravalent metals (Ge, Ti, Zr) have been hydrothermally synthesized and characterized by various spectroscopic and thermoanalytic methods. Zr-Sil. shows nearly no increase of the unit cell volume. Strong indications for an incorporation of Zr in the framework arise from a Raman band at 685 cm- 1 and a DTA peak that is about 30 K higher than for silicalite. In contrast to silicalite a symmetry change from orthorhombic to monoclinic is not observed. In Ge-Sil. a 29 Si NMR signal at -110 ppm of Si-O-Ge groups can be resolved under optimal conditions only. Combined Raman (band at 960 cm −1 ), XPS and ESR measurements of Ti-Sil. allow to distinguish between Ti isomorphously substituting Si in the framework and between extra-framework Ti

Study on the nature of aluminum in dealuminated zeolite ZSM-20

The nature of aluminum species in activated and dealuminated HZSM-20 type zeolites has been investigated by high-resolution one-dimensional solid state 27 Al MAS NMR and two-dimensional quadrupole nutation 27 Al MAS NMR spectroscopy. Additionally, IR spectra of adsorbed pyridine were recorded and the catalytic activity of samples was tested in the n-hexane conversion. From NMR experiments it is concluded that during activation and following dealumiation different kinds of 4-, 5- and 6-coordinated extraframework aluminum species are formed strongly depending on conditions of activation and dealumination. Namely, after activation monomeric extraframework Al of very high mobility or of high local symmetry (cubic) seems to appear. This sample exhibits the highest catalytic activity suggesting that monomeric or at least low-condensed extraframework Al plays an important role as active species in acid catalyzed reactions.

Physicochemical and catalytic properties of a silica-enriched zeolite ZSM-20

Zeolite ZSM-20 with and enhanced Si : Al ratio after activation has been obtained by a modification of the usual synthesis route. Despite the increased silica content the crystallinity is high and the crystal structure is nearly identical to that of the ZSM-20 zeolites reported so far. The synthesis was found to be extremely sensitive to the conditions of gel preparation, especially to pH adjustment, the temperature–time regime for hydrolysis of the organic silica source, the evaporation of the released ethanol and the formation of the silica precursors. The well crystallized synthesis product was characterized by X-ray powder diffraction, infrared spectroscopy, thermal analysis, n-hexane adsorption and temperature-programmed desorption of ammonia. The acidity of this ZSM-20 zeolite is high compared with conventional preparations of ZSM-20 zeolites or with the structurally related zeolite Y.The catalytic properties in the isomerization of m-xylene and the conversion of ethylbenzene have been studied. The isomerization of m-xylene is characterized by a high percentage of disproportionation (toluene formation) due to the high acidity. The distribution of the trimethylbenzene isomers corresponds to the thermodynamic equilibrium. Shape selectivity is obviously not operative owing to the open pore structure. Ethylbenzene is mainly dealkylated to benzene where the new ZSM-20 zeolite is capable of breaking the ethyl side chain to yield toluene.

New α-pinene isomerization catalysts

Current views of the mechanism of the catalytic isomerization of α-pinene were discussed. Catalytic systems, modifiers, process conditions, and the effect of process conditions on catalytic activity and on the target-product (camphene, dipentene) selectivities of the process were considered. α-Pinene isomerization was investigated over a Belarussian natural aluminosilicate (Al-Si RB), its derivatives modified with HCl and H3PO4, modified diatomite (Al/Dia), and bentonite (ZSM-5/Ben). The commercial aluminosilicate catalyst AS-36 was used as a reference sample. Al-Si RB was treated with 10% HCl (50–250 ml/g) or 10% H3PO4 (50–100 ml/g) at 50°C for 3 h. α-Pinene was isomerized in a round-bottom flask with a backflow condenser under flowing nitrogen. The rotational speed of the stirrer was 10 rps. The highest catalytic activity and the highest camphene selectivity were observed for Al-Si RB treated with 50 ml/g of HCl. With activated Al-Si RB and As-36, 85% α-pinene conversion at 130°C and a catalyst concentration of 0.5 wt % was attained in 5.5 and 7 h, respectively. The camphene selectivity for Al-Si RB treated with 50 ml/g of HCl was 61%, being ∼20 and ∼10% higher than in the case of AS-36 and Al/Dia, respectively. Al-Si RB retained its catalytic activity for 70 h and could be fully regenerated with acetone.

Novel nano-structured materials: synthesis and application

Novel nano-structured materials: nano-particle materials like V-SiO2, TiO2 and nano-sized pore materials like Si-MCM-41, Ti-MCM-41 and Al-MCM-41Analogues were successfully synthesised using different methods: precipitation, sol-gel, micro-emulsion and hydrothermal treatment. These obtained nano-structured materials were characterised by using different physico-chemical methods: FE-SEM, TEM, XRD, IR, UV-vis and nitrogen adsorption/desorption (BET). These materials were examined to investigate adsorptive and catalytic properties of the materials. Nano TiO2 exhibited highly photocatalytic activity in the degradation of methyl red (MR). Pure silica form MCM-41 (Si-MCM-41Analogue) was a selective absorbent for VOCs (m-xylene) removal. Ti substituted MCM-41Analogue exhibited highly photocatalytic activity in the degradation of red phenol (RP) while Al substituted MCM-41Analogue showed high catalytic cracking of petroleum residue (Bach Ho oil field – Vietnam). The obtained results are discussed and rationalised.

Novel method for doping nano TiO2 photocatalysts by chemical vapour deposition

Highly active photocatalytic Fe-doped nano TiO2 was successfully synthesised by chemical vapour deposition (CVD) method using FeCl3 as Fe source. CVD was carried out by evaporating FeCl3 at 350°C in nitrogen flow during 30–90 min. The amount of Fe incorporated into TiO2 framework is adjusted by the amount of FeCl3 used and the evaporation time. The obtained sample was characterised by X-ray diffraction (XRD), atomic absorption spectroscopy (AAS), energy dispersive X-ray spectroscopy (EDS), UV-Vis, Fourier transform-infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS). Photocatalytic activities of the samples were tested in photocatalytic decomposition of 2-propanol in liquid phase using visible light instead of UV light irradiation. Non-doped TiO2 and high Fe loading TiO2 samples showed very low photocatalytic activity, whereas the low Fe loading TiO2 sample exhibited high photocatalytic activity under visible light. The high photocatalytic activity of this sample was rationalised by the existence of defects (Ti–OH groups) as the active sites.

Study on the preparation of active support and multi-porous supported catalyst

Abstract The preparation of improved catalytic materials based on natural diatomite is reported. Aluminum incorporation by an atomic implantation methods yields supports with enhanced acidity. Nano-zeolite Y supported diatomite composite materials with a multimodao distribution of interconnected micro- meso- and macro pores were obtained by in situ crystallization. Materials were characterized by XRD, TEM, NH 3 -TPD and textural studies using nitrogen adsorption / desorption measurements. The catalytic performance was tested in the cracking of a heavy petroleum residue. The results show, that diatomite due to its large pores is a superior support for the preparation of supported catalysts for the cracking of heavy oil fraction containing bulky molecules. Atomic implantation of aluminum yields catalysts of the same catalytic performance as loading with acidic nano-sized zeolite HY.