F.R. Ribeiro

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.

Dealumination of zeolites: Part VII. Influence of the acid treatment of a HBEA zeolite on the framework composition and on the porosity

The dealumination of a HBEA sample (total and framework Si/Al ratios of 11.15 and 15.5 respectively) by treatment with a chlorhydrie acid solution was investigated under various operating conditions : HC1 concentrations of 0.1, 0.5, 1.0 and 1.5 N, temperatures of 30, 60 and 100°C, times between 5 minutes and 6 hours. Examination by IR spectroscopy of hydroxyl groups in the parent zeolite and in a series of dealuminated samples shows that the acid treatment causes a preferential elimination of the extraframework aluminium species and of transient-state framework aluminium species. A linear correlation was obtained between the wavenumber of the asymmetric stretching vibration band and the number of framework aluminium atoms of the most dealuminated samples which did not contain extraframework aluminium species. This correlation can be used to determine the composition of the zeolite framework of HBEA samples dealuminated by acid treatment. The parent zeolite presents a large mesopore volume due to intercrystalline voids created by agglomeration of the very small zeolite crystallites. The acid treatment causes a slight decrease in the microporous volume but also a significant decrease in the mesopore volume, which indicates a reorganization of the zeolite crystallites. Two periods can be distinguished in the dealumination of the HBEA framework. The initial period during which the dealumination is very rapid corresponds to the easy elimination of the Al species partially disconnected from the framework. The second period which corresponds to the elimination of the normal framework aluminium species is much slower. For both periods, the reaction order with respect to the number of framework aluminium species is close to 1 and the effect of temperature shows the existence of diffusion limitations. From die results of the kinetic study, operating conditions can be chosen so as to prepare HBEA samples with a desired framework Si/Al ratio, a good crystallinity and no extraframework aluminium species.

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.

Study of catalytic properties of SAPO-40

A series of SAPO-40 samples with Si atomic fraction from 0.06 to 0.12 has been synthesised. The study of their catalytic properties has been evaluated by comparison with SAPO-37, USHY and mordenite, using as model reaction the cracking of n-heptane. The catalytic activity of SAPO-40 samples depends on the Si content, and is probably associated with the non-uniform distribution of silicon. SAPO-40 showed a stronger acidity than SAPO-37 with a similar Si content. SAPO-40 is more resistant to deactivation by coke than the studied mordenite sample. Its selectivity for cracking products is comparable to USHY, but hydrogen transfer is lower for SAPO-40 than for USHY and mordenite.

Thermal and hydrothermal stability of the silicoaluminophosphate SAPO-40

Abstract Thermal and hydrothermal stability of SAPO-40 were studied by X-ray powder diffraction (XRD) and magic-angle spinning nuclear magnetic resonance (MAS-NMR). For the X-ray study, the hydrothermal treatments were carried out, in situ, in a high-temperature chamber. The SAPO-40 structure shows a remarkable stability to high temperatures under both dry and humid air. At high temperatures a reorganisation of silicon occurs, but the long-range order is only lost above ca. 1000°C. Changes in the XRD pattern induced by water at room temperature are completely reversible upon re-heating at 200°C, as well as the changes in the coordination and local environments of aluminium and phosphorus observed by NMR. For silicon however, some changes, that are observed upon hydration, remain after dehydration.

Development of Carbon Coatings for Cordierite Foams : An Alternative to Cordierite Honeycombs

Cordierite foams were prepared by replication of polyurethane foams and were coated with three types of carbon xerogels. The dip coating and synthesis conditions were optimized, and the coated foams were characterized exhaustively. The composition of the starting solution, coat loading, and carbonization temperature are the most important parameters determining both textural and mechanical properties. Carbon xerogel coatings obtained from aqueous solutions of resorcinol (R) and formaldehyde (F) are macro-, meso-, and microporous but present the greatest shrinkage, which causes a loss of adhesion between ceramic foams and carbon coatings. The coatings from polyfurfuryl alcohol (PFA) and RF-poly(vinyl butyral) (Butvar) resin are highly microporous and present very good adhesion even after carbonization. In all cases, coatings induce the improvement of the mechanical properties, which is related to the fact that the coating fills the defects present in the cordierite foams, thereby affecting both the rigidity and the way cracks propagate through the coated samples. These materials, due to the synergetic role of the highly porous coatings and the tortuous channels of the ceramic foams, are suitable materials for adsorption or catalytic treatments of fluids.

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.

Hydroisomerization and Hydrocracking of Alkanes. 5. Hydroisomerization and Hydrocracking of N-Hexane and N-Heptane on PtHY Catalysts. Effect of the Distribution of Metallic and ACID Sites.

The activity, the stability and the selectivity of a series of bifunctional PtHY catalysts containing 0.02 to 1.5 wt% platinum and having Si/Al atomic ratios of 3 or 9 were compared for n-hexane and n-heptane transformations at 250°C, 1 atm, pH 2 /P alkane = 9. The balance between the two functions was characterized by nPt/nA (nPt: number of accessible platinum atoms, nA: number of strong acid sites). Qualitatively the change as a function of nPt/nA of the catalytic characteristics is the same with n-hexane and with n-heptane: for low values of nPt/nA the activity per acid site is low, the stability is weak and monobranched isomers M, dibranched isomers B and cracking products C are formed directly from the reactant while for very high values the activity is optimal, the stability perfect and M, B, C are formed through a step-by-step process (“ideal” hydroisomerization catalyst). Quantitatively big differences exist. In particular in order to be “ideal”, a PtHY catalyst must have a much lower nPt/nA ratio for n-hexane isomerization. Moreover the reactivity of n-hexane is much lower than that of n-heptane. These results are interpreted in the light of the types of carbenium ions and of their rearrangement and scissions implied in n-hexane and n-heptane transformations.

An elegant way to increase acidity in SAPOs: use of methylamine as co-template during synthesis

SAPO-34 and SAPO-11 with an increased number of acid sites were synthesized by adding methylamine (MA) as co-template to tetraethylammonium hydroxide (TEAOH) or dipropylamine (DPA) respectively currently used as true templates. Along with minor changes of the final chemical and morphological properties of the as-synthesized materials, the major role of methylamine was shown to prevent a rapid silica polymerization (or to slow down its depolymerization) in the early synthesis steps, thereby favoring the progressive framework insertion of small Si oligomers that further generate an increased number of SAPO domains, with respect to Si islands.