M.F. Ribeiro

Catalytic oxidation of toluene over CuNaHY zeolites: Coke formation and removal

Abstract Catalytic deep oxidation of toluene at low concentration in air, has been studied over CuNaHY zeolites with different copper contents. The reaction has been investigated in the course of time-on-stream, at temperatures between 150 and 500°C. The effect of copper content on the conversion of toluene into CO 2 and on the formation of carbonaceous deposits (coke) retained inside the zeolite pores has been analyzed as a function of the reaction temperature. The analysis of the coke extracted from the catalysts showed that it is mainly composed of aromatic hydrocarbons and oxygenated aromatic compounds, the latter being predominant at low temperature and preferentially formed on the catalysts containing more copper. A bifunctional mechanism was proposed to explain the formation of coke over CuNaHY catalysts. Based on the differences of the nature of coke formed on NaHY and CuNaHY catalysts, and on a discussion about the coke formation and location, it was possible to conclude that copper sites are involved in both toluene and coke oxidation reactions. The increase of copper content from 1 to 8xa0wt.% promotes the oxidation of toluene and facilitates the removal of coke formed in the course of reaction.

Characterisation of CuMFI catalysts by temperature programmed desorption of NO and temperature programmed reduction. Effect of the zeolite Si/Al ratio and copper loading

Copper MFI zeolites with different Si/Al ratios and different copper loadings, prepared by ion exchange, were characterised by XRD, H2-TPR and NO TPD. The results indicated the existence of diverse copper species in CuMFI catalysts, such as isolated CU2+, Cu+ ions and CuO species, whose concentrations depends on the catalyst Si/Al ratio and copper loading. In underexchanged catalysts (catalysts with low Si/Al ratio or copper loading) in the copper mainly present as isolated Cu2+ ions, while in overexchanged catalysts (catalysts with high Si/Al ratio or copper loading) CuO species, Cu2+ and Cu+ ions were detected. CuMFI with high Si/Al ratio or high copper loading are more easily reduced than CuMFI with low Si/Al ratio or low copper loading. An easier reducibility of the copper species has an effect on the operating temperature window of the catalysts by decreasing the temperature of the maximum NO conversion. CuMFI zeolites with a high concentration of isolated Cu2+ ions exhibit the highest NO adsorption capacity. The results make evident that the isolated Cu2+ ions are the most active species for NO SCR by propene.

NO TPD and H2-TPR studies for characterisation of CuMOR catalysts The role of Si/Al ratio, copper content and cocation

Abstract NO TPD, H 2 -TPR and XRD have been used to characterise copper-exchanged mordenites with different Si/Al ratios, copper contents and cocations. The results showed that copper is mainly in the form of isolated Cu 2+ ions in CuMOR catalysts with copper exchange ≤20%, whereas at higher copper exchange CuO species are also present. These results were obtained with H and Na as cocation and were achieved by changing either the catalyst Si/Al ratio or the copper content. The data also indicate that the cocation mainly affects copper location and that copper is more easily reduced in sodium form catalysts than in protonic form. It was found that the isolated Cu 2+ ions are the most effective species for NO adsorption and the most active species for NO SCR.

An FT-IR study of NO adsorption over Cu-exchanged MFI catalysts : Effect of Si/Al ratio, copper loading and catalyst pre-treatment

Abstract NO adsorption, at room temperature, has been studied by infrared spectroscopy on copper MFI catalysts with different Si/Al ratios and copper loadings. The relative amount of the overall Cu 2+ –NO and Cu + –NO complexes has been evaluated for each catalyst, as well as that of the different Cu 2+ –NO components. ESR spectra have also been recorded in order to check the different Cu 2+ coordinations. It clearly appears that square planar copper sites predominate for high Cu/Al catalysts. A comparison with reactivity data, relative to SCR of NO by propene, under an excess of oxygen, strongly suggests that these sites are involved in the considered reaction.

Selective catalytic reduction of NO with propene over CuMFI zeolites: dependence on Si/Al ratio and copper loading

Abstract Selective catalytic reduction of nitrogen oxide by propene in an oxidising atmosphere was studied on several CuMFI catalysts with different Si/Al ratios (11 ⩽ Si/Al ⩽ 100) and different copper loadings (between 0 and 5.5 wt.-%). From the results it was observed that the influence of zeolite Si/Al ratio on CuMFI catalytic activity for NO SCR by propene is dependent on the catalyst copper loading. Furthermore, the effect of catalyst copper loading on catalytic performance depended on the catalyst Si/Al ratio. The results also demonstrated that CuMFI catalysts with different Si/Al ratios and copper loadings, but with the same Cu/Al ratio and, therefore, the same copper exchange level have similar catalytic activity profiles for NO SCR. It was further observed that not all Cu cations exchanged into MFI catalysts have equivalent catalytic activity for NO SCR, which made the existence of different copper environments on CuMFI catalysts evident, isolated Cu 2+ ions being the most active species for NO SCR by propene. Moreover, the results showed an improvement of the CuMFI catalytic activity at low temperatures by increasing the catalyst copper exchange level and, consequently, decreasing the number of Bronsted acid sites, which can be performed either by increasing the zeolite Si/Al ratio or copper loading.

Deactivation of CuMFI catalysts under NO selective catalytic reduction by propene: influence of zeolite form, Si/Al ratio and copper content

Deactivation of CuMFI catalysts under NO selective catalytic reduction (SCR) by propene in both the absence and the presence of water was investigated as a function of zeolite form, Si/Al ratio and copper content. It was verified that the CuMFI deactivation extent is higher on H-form zeolite compared to Na-form and decreases when the copper exchange level increases, which can be achieved either by increasing the zeolite Si/Al ratio or the copper content. Furthermore, the results indicated that the catalyst deactivation is mainly due to a change in copper species rather than in MFI structure.

Influence of cocation on catalytic activity of CuMOR catalysts for NO SCR by propene. Effect of water presence

Catalytic activity of CuMOR catalysts for NO selective catalytic reduction by propene was investigated as a function of the cocation (H, Na, Cs, Mg and Ba) present in the zeolite. It was verified that the cocation plays an important role in the CuMOR activity, its effect being more pronounced at low temperatures. Moreover, the results suggest that we can prevent the catalyst deactivation by choosing the appropriate cocation.

Multiple-quantum 27Al MAS n.m.r. spectroscopy of microporous AlPO-40 and SAPO-40

Two-dimensional triple and quintuple-quantum 27 Al magic-angle spinning (MQ MAS ) n.m.r. spectra of microporous AIPO-40 and SAPO-40 have been recorded at 9.4 and 14.1 T. The resolution of the 5Q spectra of AIPO-40 is amazing, at least 11 resonances being resolved. The 3Q spectra further reveal the presence of 2 other much fainter peaks. Because the AIPO-40 aluminium sites have different quadrupole coupling constants the 9.4 T spectra are slightly better resolved than the 14.1 T spectra, suggesting that in MQ n.m.r. it may sometimes be advantageous to work at lower magnetic fields. The 3Q spectra of SAPO-40 are poorly resolved displaying a main broad peak and two faint resonances. This is because the introduction of even a small amount of Si into the framework of AIPO-40 generates a distribution of Al sites and a dispersion of chemical shifts. MQ 27 Al MAS n.m.r. together with 31 P MAS n.m.r. evidence discard an orthorhombic, Pccn , and monoclinic, P112/n , structure for as-prepared or calcined dehydrated AlPO-40, suggesting a space group with even lower symmetry that proposed to date.

Copper-exchanged mordenites as active catalysts for NO selective catalytic reduction by propene under oxidising conditions: Effect of Si/Al ratio, copper content and Brönsted acidity

Abstract CuMOR catalysts with different Si/Al ratios and copper contents, prepared from the acid and sodium forms, were studied in NO reduction with propene in the presence of excess oxygen. It was observed that the influence of zeolite Si/Al ratio on CuMOR catalytic activity for NO SCR by propene depends on the catalyst copper content, the reverse being also true. For mordenites prepared from the acid form, it was shown that the most active catalyst must have ∼60% of copper exchange, whereas for those prepared from the sodium form, the maximum catalytic activity is obtained for catalysts with 20% of copper exchange. Moreover, it was observed that at low copper-exchange levels, the catalyst prepared from the sodium form exhibits much higher activity than those prepared from the acid form. Although, when the copper-exchange level increases, the effect of zeolite form is less pronounced. Thus, it was indicated that Bronsted acidity does not promote the NO selective reduction by propene.

Influence of the treatment of mordenite by ammonium hexafluorosilicate on physicochemical and catalytic properties

Different samples were prepared by treating a NH 4 MOR sample (Si/Al = 5) with ammonium hexafluorosilicate. The dealumination of the outer surface of mordenite crystallites was more pronounced than that of the bulk. A decrease in the adsorption rate and capacity for organic molecules and in activity and stability for m -xylene transformation was observed. Furthermore, the isomerization selectivity was improved because of inhibition of desorption of bulky products of m -xylene disproportionation. All of these observations can be related to a dealumination of the mordenite limited to the pore mouth with deposit, at the entrance of channels, of aluminum species extracted from the framework.