Izabela Sobczak

Oxidative properties of niobium-containing mesoporous silica catalysts

Abstract Oxidative properties of MCM-41 molecular sieves containing niobium in the framework or/and in the extra framework positions were studied by means of H2-TPR, FTIR+NO, ESR techniques and were tested in the oxidation of dibutyl sulphide with hydrogen peroxide. The application of various Nb-sources in the synthesis of NbMCM-41 materials allows the estimation of a role of oxalate ions in the generation of active centres. The activity of Nb-containing mesoporous molecular sieves in the oxidation of thioethers with H2O2 requires the presence of Nb–O− species in the framework. The growth of a concentration of niobium (from Si/Nb=32 to Si/Nb=16) results in the decrease of the number of active centres due to the release of oxygen during the activation procedure. Niobium species in the extra framework positions of MCM-41 catalysts are not active in the dibutyl sulphide oxidation with H2O2 unless they are leached to the solution. Oxidative centres on the catalyst surface cause the formation of nitrite/nitrate species after the adsorption of NO.

Nb-containing mesoporous molecular sieves — a possible application in the catalytic processes

Abstract Nb-containing MCM-41 mesoporous molecular sieves were modified with ammonium and copper cations and characterised by ESR, FTIR and H 2 -TPR techniques. The prepared materials were tested in the following catalytic reactions: NO decomposition, reduction of NO with NH 3 , hydrosulphurisation of methanol, and oxidation of thioethers with hydrogen peroxide. The oxidising character of the NbMCM-41 mesoporous molecular sieves was revealed, which induces their application as very active and selective catalysts in the oxidation of thioethers with H 2 O 2 to sulphoxides. HNbMCM-41 samples, which exhibit Lewis acidity after dehydroxylation, are selective catalysts in the methanethiol synthesis in the reaction between methanol and hydrogen sulphide. Copper modified NbMCM-41 sieves indicate the higher reducibility of cupric cations than that observed when AlMCM-41 matrix for copper is used. The higher reducibility of cupric ions does not cause a higher activity in the catalytic NO decomposition.

Physico-chemical and catalytic properties of MCM-41 mesoporous molecular sieves containing transition metals (Cu, Ni, and Nb)

Publisher Summary This chapter describes recent works concerning the physicochemical and catalytic properties of Nb-containing mesoporous sieves. The behavior of NbMCM-41 materials is compared with the one of AlMCM-41 molecular sieves. Moreover, the chapter discusses how the mesoporous matrices (i.e. aluminosilica (AlMCM-41) and niobiosihca (NbMCM-41)) affect the properties of the protons-, nickel-, and copper-exchanged materials. The choice of the transition metals incorporated to the mesoporous sieves depended on the fixture use of the materials either as sorbents or catalysts in the transformation of sulfur compounds (Nb- and Ni-materials), or in the catalytic redox reactions (Cu- and Nb-sieves).

Effect of sulfur dioxide on nitric oxide adsorption and decomposition on Cu-containing micro- and mesoporous molecular sieves

NO decomposition was studied at different temperatures on copper-exchanged ZSM-5, AlMCM-41 and NbMCM-41 molecular sieves. Cu-ZSM-5 zeolites presented the highest activity. SO2 poisoning was also performed and Cu–NbMCM-41 was found to be more resistant. IR results of NO and SO2 coadsorption either at room temperature or at 573 K show evidence that sulfate formation occurred at 573 K and partially prevented NO adsorption on Cu2+ in square planar structure in Cu-ZSM-5. Sulfation of Cu–NbMCM-41 was quite low due to niobium incorporated into the lattice. By contrast, niobium present in the extra-lattice position in CuNb-ZSM-5 and CuNb–AlMCM-41 did not protect the catalyst from sulfation. H2-TPR results suggested that sulfates were formed on copper sites. IR spectra after treatment under SO2 + O2 at 673 K indicated that sulfated species were covalently bonded, their structure varying according to the nature of the support.

Physico-chemical and catalytic properties of Ni-containing mesoporous molecular sieves of MCM-41 type

Dehydrated Ni-containing AlMCM-41 and NbMCM-41 mesoporous molecular sieves were characterised by means of N 2 adsorption, XRD, H 2 -TPR, ESR, NO adsorption combined with FTIR spectroscopy, and study of their activity in the dehydrosulfurisation (DHS) of thiols and sulfides. NbMCM-41 materials change porosity after the Ni-cation exchange. Both Ni + and Ni 2+ species were identified on Ni-cation exchanged materials and NiO and Ni 2+ species were found on the impregnated sample. Ni + species seems to play a key role in DHS.

The possible use of mesoporous molecular sieves for deodorisation

Mesoporous molecular sieves of MCM-41 type, thanks to their very high sorption capacity, are useful sorbents for odours such as organic sulfur compounds. Two processes for the regeneration of sorbents are proposed in this paper: i) dehydrosulfurisation (DHS) and ii) oxidation with H2O2. They were studied using butanethiol and dibutyl sulfide as the representatives of the sulfur compounds. In the DHS reaction, NiAlMCM-41 catalysts are the best due to their high activity and selectivity to H2S and C4 hydrocarbons. The Ni-cation exchanged material is much more active and selective to the desired products than the Ni- impregnated sample. The high activity in the oxidation of dibutyl sulfide to sulfoxide with H2O2 has been found on Nb-containing materials. These catalysts are easily regenerated recovering the previous activity.

NO adsorption and decomposition on Cu-containing mesoporous molecular sieves - comparison with CuZSM-5

Nitric oxide adsorption and decomposition on CuAlMCM-41 mesoporous molecular sieves and CuZSM-5 zeolites (as reference materials) were studied by means of ESR, FTIR and a catalytic test. It was indicated that the Cu-sites reducibility and Cu+(NO) complex stability controlled by the Cu/Al/Si compositon are important parameters in NO decomposition on mesoporous materials. The Cu+(NO) species was well detectable only on the overexchanged CuAlMCM-41-32-132 mesoporous sieve and various CuZSM-5 zeolites. On the other samples Cu2+(NO2−)(NO) was the main complex registered and it was active in NO decomposition.

Effect of hydrogen sulphide on nitric oxide adsorption and decomposition on Cu-containing molecular sieves

Abstract NO adsorption and decomposition were studied on sulphided and sulphated copper-exchanged ZSM-5, AlMCM-41 and NbMCM-41 molecular sieves. FTIR, temperature-programmed reduction (H2-TPR) techniques, adsorption, and catalytic measurements were applied. H2S adsorption drastically decreases the activity of copper-containing micro- and mesoporous materials studied in NO decomposition due to their sulphidation. This decrease is higher than that observed after SO2 treatment and is assigned to the formation of sulphide species. H2S is strongly chemisorbed on all copper active sites. Oxidation of sulphide species leading to the formation of sulphates does not increase the activity in the decomposition of NO, suggesting that some sulphided species are not oxidised.

Novel AuNbMCM-41 catalyst for methanol oxidation

The synthesis, characterization and catalytic application of bifunctional gold and niobium containing MCM-41 molecular sieves are reported for the first time. It is shown that Au is better dispersed when introduced during the synthesis than if it is deposited via impregnation. Moreover, it is evidenced that the introduction of Nb besides gold in the MCM-41 framework significantly shifts the oxidation of methanol towards formaldehyde (industrially important compound) at relatively low reaction temperature.

Gold and gold-iron modified zeolites--towards the adsorptive deodourisation.

Zeolites exhibiting different structures (Y, Beta, and ZSM-5) were modified with gold and iron and applied for odour adsorption from the air containing dibutyl sulphide (Bu(2)S) used as a representative odour producing compound. The structure of the zeolites used determines the rate of adsorption (higher on Y type zeolites and smaller on two other zeolites), whereas hydrophilicity affects the selectivity towards Bu(2)S adsorption increasing in the order: Y<Beta<ZSM-5. For the same zeolite structure, Bu(2)S adsorption selectivity depends on the total acidity of zeolites which increases after iron modification. The texture and surface properties of the modified zeolites were studied by XRD, XPS, UV-vis, TEM, pyridine adsorption and FTIR, test reactions (acetonylacetone cyclisation, isopropanol decomposition).