A Cu/Al-MCM-41 mesoporous molecular sieve: application in the abatement of no in exhaust gases

Abstract

Propane oxidation and reduction of NO to N2 with propane under oxidative conditions on a Cu-Al-MCM-41 mesoporous molecular sieve and Cu-ZSM-5 zeolites were studied. Both types of catalysts were prepared by ion exchange in aqueous solutions of copper acetate and characterised by X-ray diffraction (XRD), nitrogen sorption measurement, diffuse reflectance ultra-violet spectroscopy (DRS-UV), diffuse reflectance infra-red Fourier transform spectroscopy (DRIFTS) of the adsorption of CO on Cu+ and temperature-programmed reduction with hydrogen (H2-TPR). The NO reduction was performed between 200 and 500 ºC using a GHSV = 42,000 h-1. H2-TPR data showed that in the prepared Cu-Al-MCM-41 all the Cu atoms are on the surface of the mesopores as highly dispersed CuO, which results in a decrease in specific surface area and in mesopore volume. H2-TPR together with DRIFTS data provided evidence that in Cu/ZSM-5 catalysts, Cu atoms are found as two different Cu2+ cations: Cualpha2+ and Cubeta2+, which are located on charge compensation sites, and their thermo-redox properties were different from those of Cu atoms in Cu-Al-MCM-41. The specific activity of the Cu2+ exchangeable cations in Cu-ZSM-5, irrespective of their nature, was much greater than that of the Cu2+ in Cu-Al-MCM-41, where they are found as CuO.

Cu-MCM-41; Cu-ZSM-5; NO reduction; Propane oxidation

REACTORS ENGINEERING AND CATALYSIS

A Cu/Al-MCM-41 mesoporous molecular sieve – Application in the abatement of no in exhaust gases

M. S. Batista^I; R. A. A. Melo^II; M. Wallau^I; E. A. Urquieta-GonzálezI, ^*

^IDepartamento de Engenharia Química, Universidade Federal de São Carlos, Phone [phone], Fax [phone], Cx. P. 676, CEP 13565-905, São Carlos - SP - Brazil. E-mail: [e-mail]

^IILaboratório de Catálise e Desenvolvimento de Materiais, Centro Universitário do Sul de Minas, Av. Cel. José Alves 256, CEP 37010-540, Varginha - MG, Brazil

ABSTRACT

Propane oxidation and reduction of NO to N₂ with propane under oxidative conditions on a Cu-Al-MCM-41 mesoporous molecular sieve and Cu-ZSM-5 zeolites were studied. Both types of catalysts were prepared by ion exchange in aqueous solutions of copper acetate and characterised by X-ray diffraction (XRD), nitrogen sorption measurement, diffuse reflectance ultra-violet spectroscopy (DRS-UV), diffuse reflectance infra-red Fourier transform spectroscopy (DRIFTS) of the adsorption of CO on Cu⁺ and temperature-programmed reduction with hydrogen (H₂-TPR). The NO reduction was performed between 200 and 500 ºC using a GHSV = 42,000 h^-1. H₂-TPR data showed that in the prepared Cu-Al-MCM-41 all the Cu atoms are on the surface of the mesopores as highly dispersed CuO, which results in a decrease in specific surface area and in mesopore volume. H₂-TPR together with DRIFTS data provided evidence that in Cu/ZSM-5 catalysts, Cu atoms are found as two different Cu²⁺ cations: Cu_a²⁺ and Cu_b²⁺, which are located on charge compensation sites, and their thermo-redox properties were different from those of Cu atoms in Cu-Al-MCM-41. The specific activity of the Cu²⁺ exchangeable cations in Cu-ZSM-5, irrespective of their nature, was much greater than that of the Cu²⁺ in Cu-Al-MCM-41, where they are found as CuO.

Keywords: Cu-MCM-41; Cu-ZSM-5; NO reduction; Propane oxidation.

INTRODUCTION

With the objectives to minimise the amount of nitrogen oxides (NO_x) emitted into the atmosphere and to develop a catalyst which could have better performance in the conversion of this type of pollutant into N₂, different metal-exchanged microporous molecular sieves have been studied. In this field, the first work was carried out by Iwamoto (1994), who found that Cu-exchanged ZSM-5 zeolites catalysts were active in the reduction of NO to N₂ with small quantities of light hydrocarbons such as C₂H₄ and C₃H₆ and an excess of O₂. MFI, MOR and FAU zeolites are the most frequently studied aluminium-silicate microporous structures (Kikuchi and Yogo, 1994).

In the nineties, with the synthesis of the MCM-41 mesoporous molecular sieve (Beck et al., 1992), several studies were directed towards application of these types of solids in separation and catalysis processes (Melo et al., 2000; Trong On et al., 2001). The most interesting characteristic of these materials, which have mesopores with diameters between 2.0 and 10.0 nm, is their large specific surface areas, with values on the order of 1,000 m²/g. In contrast with other mesoporous materials, like activated carbon and aerogels, the ordered mesoporous (metallo)silicates described by Beck et al. (1992) have a narrow pore size distribution. For example, a typical MCM-41 material shows exclusively mesopores of around 4 nm with a pore size distribution with a width at half height of approximately 0.4 nm (Beck et al., 1992).

The appeal of the formation mechanism of these mesoporous silicates is its simplicity, which easily permits modification of pore diameters and the composition of the solid obtained. The templates used in MCM-41 synthesis are surfactants characterised by a long hydrophobic chain and a hydrophilic head group. In an aqueous reaction medium, these surfactants form worm-like micelles, which by self-aggregation generate arrays, e.g. hexagonal arrays in the case of MCM-41. In the presence of adequate inorganic oxide precursors, such as SiO₂, petrification of the liquid crystal structure occurs by precipitation of these oxides between the cylindrical micelles. Removal of surfactant molecules by calcination or solvent extraction produces ordered porous materials, since pore diameters can be tailored by suitable choice of the surfactant (Beck et al., 1992).

In this context and with the aim to verify the use of this type of mesostructured material in the treatment of NO emissions, in this work a |Cu|[SiAl]-MCM-41 mesoporous material was prepared and its properties and catalytic behaviour compared with those of |Cu|[SiAl]-MFI catalysts, which are usually used as reference in the selective catalytic reduction of NO with hydrocarbons.

EXPERIMENTAL

Denominations of the samples prepared in this study were based on IUPAC recommendations (McCusker et al., 2001) as |M_x|[Si_yAl]-ZZZ, where M represents the hosted metal specie, x the metal/Al ratio, y the Si/Al ratio and ZZZ the structure type, i.e. MFI for microporous ZSM-5 and MCM-41 for the mesoporous material.

The precursor |Na₁|[Si₂₃Al]-MCM-41 mesoporous molecular sieve and |Na₁|[Si_yAl]-MFI zeolite were prepared in accordance with the procedures utilized by Melo et al. (2001) and Batista (1997), respectively. |Cu_x|[Si_yAl]-MCM-41 and |Cu_x|[Si_yAl]-MFI catalysts were prepared by ion exchange at room temperature, utilising a solution of copper acetate (0.033 mol/L). After ion exchange the samples were filtered, washed with deionised water and dried at 110 ºC during 12 h. A reference sample was also prepared by mixing copper oxide and the precursor |Na₁|[Si₁₁Al]-MFI. The solid mixture was subsequently calcined at 500 ºC for 2 h and the solid obtained was denominated as |CuO_0.15|[Si₁₁Al]-MFI. The CuO used was prepared by calcination of copper acetate (Aldrich 99 %).

The samples obtained were characterised by X-ray diffraction (XRD), atomic absorption spectroscopy (AAS), nitrogen sorption measurements, UV-diffuse reflectance spectroscopy (DRS-UV), analysis of adsorbed CO by diffuse reflectance IR spectroscopy (DRIFTS) and temperature-programmed reduction by H₂ (H₂-TPR). The XRD analyses were performed on a Siemens D500 diffractometer, using nickel-filtered Cu-?_a radiation and a goniometer velocity of 2° (2Q)/min. The N₂ sorption measurements of the sample previously treated at 150 ºC for 2 h under vacuum were made at the temperature of liquid N₂ on Nova-1200 equipment (Quantachrome Corporation). The specific surface area (S_BET) of |Na₁|[Si₂₃Al]-MCM-41 and |Cu_5.7|[Si₂₃Al]-MCM-41 was calculated by the BET method (Brunauer et al., 1938) from the amount of adsorbed N₂ in the range of 0.06 < P/P₀ < 0.24. The mean diameter of the mesopores (d_mp) in |Cu_5.7|[Si₂₃Al]-MCM-41 was determined by the model of BJH (Barret et al., 1951).

DRS-UV analyses were conducted at room temperature on a Varian Cary 5G spectrometer equipped with a diffuse reflection cell. A Teflon sample holder, a quartz window and a Teflon standard were used as reference material. Prior to analysis, the samples were dried at 110 ºC for 24 h. The resultant reflections (R) were treated with the function of Schuster-Kubelka-Munk, which is represented by the equation F(R) = 100(1-R)²/2R. For the DRIFTS analysis, an infrared spectrometer Nicolet 750 with a spectral resolution of 4 cm^-1 was used. The samples were pretreated in situ at 450 ºC under O₂ flow (2 % in N₂, v/v) and subsequently cooled to 25 ºC. Before the adsorption of CO, Cu²⁺ ions were reduced to Cu⁺ at 220 ºC using a H₂ flow (25% in N₂, v/v). The CO (99.997 %) was adsorbed at 25 ºC, feeding in pulses of CO until the surface was saturated (P_CO = 28 Torr) and then removing the excess under N₂ flow. During the DRIFTS experiment, 128 scans were accumulated. The band obtained at 2157 cm^-1 was deconvoluted with the software Origin 5.0 Peak Fitting using a model of the voigt curve and a constant width at half height.

H₂-TPR analyses were carried out on Micromeritics 2705 equipment having a thermal conductivity detector. An H₂ flow (5% in N₂, v/v, 30 mL/min) and a heating rate of 10 ºC/min were used. Prior to analysis the sample (100 mg) was treated at 200 ºC under He flow for 1 h.

|Cu_5.7|[Si₂₃Al]-MCM-41 and |Cu_x|[Si_yAl]-MFI catalysts were evaluated in the reduction of NO to N₂ using propane as reducing agent. Prior to reaction, the catalyst was activated for one hour under air flow at 500 ºC and subsequently cooled to room temperature. The feed containing an excess of O₂ (simulating an exhaust gas mixture) had a composition of 0.30 % NO, 0.32 % C₃H₈ and 1.8 % O₂ balanced in He (v/v). The reaction temperature was varied between 200 and 500 ºC and a gas space velocity (GHSV) of 42,000 h^-1, which was calculated considering the total gas flow, was used. To avoid hot spots, a mass of 50 mg of dry catalyst was homogeneously mixed with 150 mg of a-quartz. The products were analysed using a gas chromatograph (Shimadzu-GC-17A) equipped with a flame ionisation detector (FID) and a thermal conductivity detector (TCD) and three chromatographic columns, one capillary (Al₂O₃/KCl, 30 m × 0.32 mm) and two packed, a Hayesep D (3 m × 1/8") and a Chromossorb 102 (5 m × 1/8").

The activity of the catalysts in propane oxidation was expressed in terms of its total conversion, taking into account the carbon balance during the reaction. In the case of the reduction of NO, its conversion to N₂ was calculated using the following equation: X_NO (%) = 2 [N₂] × 100/[NO]_o, where [N₂] is moles of N₂ formed and [NO]_o is moles of NO fed into the reactor.

RESULTS AND DISCUSSION

Characterisation

In Table 1 the Cu content (% Cu w/w) and the Cu/Al ratio of the prepared |Cu_x|[Si_yAl]-MFI and |Cu_5.7|[Si₂₃Al]-MCM-41 catalysts are shown. It can be verified that |Cu_0.46|[Si₁₁Al]-MFI and |Cu_0.28|[Si₂₃Al]-MFI samples have lower Cu contents than those corresponding to the maximal ion-exchange capacity (when the ratio of charge- compensating Cu²⁺ per framework Al is 0.5). However for the |Cu_5.7|[Si₂₃Al]-MCM-41 mesoporous molecular sieve, which was obtained utilising the same ion-exchange procedure, the Cu²⁺/Al ratio was much higher than 0.5. The different behaviour referred to is probably due to the large hydration sphere of Cu²⁺ cations, which hampers adsorption onto the micropores of ZSM-5 but not onto the mesopores of MCM-41. The Cu²⁺/Al ratio of 5.7 observed for Cu/MCM-41 suggests the presence of CuO in the solid, while for Cu/ZSM-5, Cu²⁺ cations on ion-exchange sites seem to be more likely.

The XRD patterns for calcined |Na₁|[Si₂₃Al]-MCM-41 and activated |Cu_5.70|[Si₂₃Al]-MCM-41 (Figure 1a) and |Cu_0.46|[Si₁₁Al]-MFI (Figure 1b) are typical of the molecular sieves studied (Beck et al., 1992; Treacy et al., 1996) and do not indicate the presence of other phases. Although solid-state ion exchange might have occurred in |CuO_0.15|[Si₁₁Al]-MFI, the presence of reflections of CuO whit their principal diffraction peaks at 35.7 and 38.5° (2Q) (see Figure 1b), suggests that in this sample Cu²⁺ cations on ion-exchange sites are unlikely. On the other hand, with XRD no crystalline CuO is observed in |Cu_0.46|[Si₁₁Al]-MFI, suggesting that in this catalyst, most of the Cu cations might be located on ion-exchange sites. Although the high Cu/Al ratio in |Cu_5.7|[Si₂₃Al]-MCM-41 strongly indicates the presence of non-ionic copper oxide species, in its XRD pattern it is not possible to observe the presence of peaks corresponding to copper oxide. However, this result does not exclude the possibility that this oxide may be present as small CuO crystals with diameters < 3 nm (Carniti et al., 2000), which must be finely dispersed on the mesopore walls.

The decreased intensity of the XRD reflections of MCM-41 observed after ion exchange (Figure 1a) is due to the adsorption of the radiation by the deposited Cu species rather than to structural degradation. This is also suggested by the nearly unchanged unit cell parameter a₀ of the hexagonally mesoporous arrangement of MCM-41 (Beck et al., 1992) (Table 2) and by the decrease in intensity of the broad peak around 23° (2Q), typical of amorphous material, which should be increased by structural degradation. The presence of CuO species deposited on the internal pore surface of the MCM-41 structure is also indicated by the increase in wall thickness (e) and by the decrease in mesopore area (S_p), total pore volume (V_T) and mesopore volume (V_mp) of |CuO_5.7|[Si₂₃Al]-MCM-41, as can be seen from data given in Table2.

The results of H₂-TPR (Figure 2) reveal that in |Cu|[Si₂₃Al]-MCM-41 as well as in |CuO_0.15|[Si₁₁Al]-MFI the Cu (II) cations are reduced in one step, as is typical for CuO, thus confirming that this compound is practically the only Cu species present in these samples and that no important solid-state ion exchange occurred after the thermal treatment applied in the samples discussed. The presence of CuO crystals in |Cu_5.7|[Si₂₃Al]-MCM-41 that are smaller than those in |CuO_0.15|[Si₁₁Al]-MFI is indicated by the decrease in reduction temperature, which is in accordance with the absence of X-ray reflections attributable to CuO mentioned above.

As can be seen in Figure 2, the H₂-TPR profiles show three reduction peaks for Cu_0.46|[Si₁₁Al]-MFI and Cu_0.28|[Si₂₃Al]-MFI catalysts. In accordance with the results reported by Wichterlová et al. (1997), we attributed these peaks to the step-wise reduction of two different kinds of Cu cations (Cu_a and Cu_b) as demonstrated in Table 3, Cu_a has a high positive charge density and therefore is more difficult to be reduced, and is thus found on ZSM-5 zeolites with a low Cu content. On the other hand, Cu_b has a low positive charge density, and therefore is easier to be reduced, and is thus found on ZSM-5 zeolites with a high Cu content. Taking into account the above statements, we attributed the first reduction peak at around 210 ºC to the reduction of Cu_a⁺² and Cu_b⁺² to Cu_a⁺¹ and Cu_b⁺¹, respectively. The reduction of the latter cations occurred at around 380 ºC for Cu_b⁺¹ and at around 580 ºC for Cu_a⁺².

For all the samples analysed the ratio of consumed H2 to Cu atom (H2/Cu) was nearly one, indicating that all the Cu atoms were effectively reduced. The reduction of Cua2+ or Cub2+ in two steps, observed in sample |Cu0.46|[Si11Al]-MFI, was confirmed by DRIFTS (Figure 3), by which the behaviour of the adsorption of CO on the |Cu0.46|[Si11Al]-MFI catalyst was verified. The resulting spectrum has a band at 2157 cm-1, corresponding to the adsorption of CO on Cu+ cations (Hadjiivanov and Knözinger, 2000).

The deconvolution of the band at 2157 cm-1 is shown in Figure 3b. As can be seen, that band is a consequence of the superposition of two others, a more intense band at 2157 cm-1 and another at 2145 cm-1. Taking into account the H2-TPR data discussed previously, these bands can be attributed to Cub+ and Cua+, respectively. The Cub+-CO species has a higher electron density in the d orbital than the Cua+-CO species, resulting in an increase in the occupation of the 2 p* orbital and in a weakening of the C=O bond for the former, and thus a shifting of the band to a lower frequency (band at 2145 cm-1 in Figure 3b).

The relative amount of each of these Cu+ cations, obtained from the relative intensity of its respective band in the deconvoluted DRIFTS spectrum, was 23 % for Cua2+ and 77 % for Cub2+, values very similar than those obtained from H2-TPR data (Table 2).

The N₂ adsorption and desorption isotherms for |Na₁|[Si₂₃Al]-MCM-41 and |Cu_5.70|[Si₂₃Al]-MCM-41 samples are shown in Figure 4a, while in Figure 4b the distribution of mesopore diameters as a function of volume of adsorbed N₂ is shown. In Figure 4a it can be observed that both samples have the textural characteristics of mesoporous solids (Janicke et al., 1994). The isotherms observed are of type IV and have a visible step for a relative pressure between 0.2 and 0.4, indicating the condensation of N₂ in the primary mesopores, as is typical for the case of ordered mesopores (Cui et al., 1997). However, this condensation was lower in the |Cu_5.70|[Si₂₃Al]-MCM-41 sample, thus suggesting the presence of precipitated CuO on the mesopore walls. Therefore, the reduction in specific surface area (S_BET) of 25 %, as well as the reduction in mesopore surface (S_P) and primary mesopore volume (V_mp) observed for |Cu_5.70|[Si₂₃Al]-MCM-41 (Table 2), compared to |Na₁|[Si₂₃Al]-MCM-41, is also related to its high Cu content (Table 1). This fact is also evident from the diminution in pore diameter, as can be observed in Figure 4b. These results are in accordance with those published by Minchev et al. (2001), who found that the surface area of |Cu_x|[Si_yAl]-MCM-41 mesoporous solids is influenced by Cu content and by the method used during Cu impregnation.

In Figure 5 the DRS-UV spectra for |Cu_5.7|[Si₂₃Al]-MCM-41 and |Cu_x|[Si_yAl]-MFI samples are shown. It can be observed that irrespective of the type of solid, the Cu-containing samples have bands at 215, 260 and 750 nm. For |Na₁|[Si₂₃Al]-MFI, only the first band at 215 nm was observed; therefore it was attributed to structural Al-O units in this material. However, the intensity of this band increases with the increase in Cu content in the sample. In accordance with data published by Teraoka et al. (2000), irrespective of the symmetry of Cu²⁺cations, the bands at 215 and 260 nm, correspond to the charge transfer transition between the O and the metal (O ® Cu²⁺). The band at around 750 nm corresponds to d-d electron transitions of Cu²⁺ hexagonally coordinated by water molecules and O atoms found in the framework of the molecular sieves studied (Itho et al., 1994). In accordance with Moretti et al. (2001), this band disappears when |Cu_x.|[Si_yAl]-MFI-type solids are treated under vacuum at 500 ºC. As can be seen in Figure 5a, the band at around 890 nm confirms the presence of CuO in |Cu_5.7|[Si₂₃Al]-MCM-41 (Dossi et al., 1999).

In Figure 6a the results for the selective catalytic reduction of NO to N₂ using propane as reducing agent on |Cu_x.|[Si_yAl]-MFI, |Cu_5.7|[Si₂₃Al]-MCM-41 and |CuO_0.15|[Si₁₁Al]-MFI catalysts are shown. For |Cu_x|[Si_yAl]-MFI samples, the conversion of NO was a function of Cu content in the solid (Table 1). Thus, the |Cu_0.46|[Si₁₁Al]-MFI catalyst with the highest Cu content had a higher conversion than |Cu_0.28|[Si₂₃Al]-MFI.

The low conversion of NO to N₂ (Figure 6a) and of propane (Figure 6b) on |CuO_0.15|[Si₁₁Al]-MFI indicates that CuO does not have significant activity for these reactions and that in Cu-based catalysts the Cu cationic species located in extra-framework exchange positions are responsible for the catalytic activity.

This behaviour can be best observed by comparing the activity of |Cu_5.7|[Si₂₃Al]-MCM-41 and |Cu_0.28|[Si₂₃Al]-MFI samples (Figure 6), which have very similar values for NO conversion. Although |Cu_0.28|[Si₂₃Al]-MFI has a much lower copper content than |Cu_5.7|[Si₂₃Al]-MCM-41 (Table 1), it has Cu atoms, as evidenced by the H₂-TPR results (Figure 2), as cationic species which are compensating the negative charge of the zeolite structure. In contrast, the H₂-TPR results showed that in |Cu_5.7|[Si₂₃Al]-MCM-41 all the Cu atoms are in the form of CuO (Figure 2). On the other hand, the activity of the latter sample was greater than that of |CuO_0.15|[Si₁₁Al]-MFI, with this behaviour being attributed to the higher and better dispersed amount of this oxide on the surface of the mesopores.

In Figure 7 the specific activities of the Cu atoms in the catalysts studied are shown for the oxidation of propane (Cu(spAc)_propane) and for the reduction of NO to N₂ (Cu(spAc)_NO), calculated as the number of converted molecules per Cu atom per hour. As can be observed, the data for Cu(spAc)_NO (Figure 7a) and Cu(spAc)_propane (Figure 7b) confirm the discussion of the results plotted in Figure 6, verifying that the Cu atoms in |Cu_x|[Si_yAl]-MFI catalysts, irrespective of their nature (Cu_a²⁺ or Cu_b²⁺), have greater specific activity in both reactions studied than those obtained for Cu atoms in |Cu_5.7|[Si₂₃Al]-MCM-41 and |CuO_0.15|[Si₁₁Al]-MFI. Moreover, in the latter catalyst the Cu atoms have practically null specific activity in the reactions discussed.

In Figure 7 it can be further observed that for a space velocity (GHSV) of 42,000 h^-1 and for temperatures higher than 400 ºC, the |Cu_0.28|[Si₂₃ Al]-MFI sample has higher values of Cu(spAc)_propane and Cu(spAc)_NO than |Cu_0.46|[Si₁₁Al]-MFI. However, under these operating conditions, |Cu_0.46|[Si₁₁Al]-MFI has propane conversions of 100%, and therefore its specific activity remains constant up to 400 ºC (Figure 7b). Increasing the space velocity from 42,000 h^-1 to 84,000, the Cu(spAc)_propane on |Cu_0.46|[Si₁₁Al]-MFI increased from 25 to 48.5 at 400 ºC and to 58.7 at 450 ºC (Table 4). These values remained practically constant when the space velocity was changed to 168,000 h^-1, indicating that these activity values can be considered as maximal for that catalyst. For the |Cu_0.28|[Si₂₃Al]-MFI catalyst, which has a lower Cu content, very similar values of Cu(spAc)_propane at GHSV=42,000 h^-1 were obtained (Table 4).

CONCLUSIONS

H₂-TPR and DRS-UV data demonstrated that in |Cu_5.7|[Si₂₃Al]-MCM-41 the Cu atoms are in the form of CuO dispersed on the surface of the mesopores, which results in a decrease in peak intensity in the XRD pattern and in a reduction in specific surface area and mesopore volume. On the other hand, data from H₂-TPR and DRIFTS on the adsorption of CO on Cu⁺ provided evidence that, depending on Si/Al ratio and Cu content in the zeolite, the copper atoms in Cu-exchanged ZSM-5 zeolites (|Cu_x.|[Si_yAl]-MFI), coexist as two different Cu²⁺ cations (Cu_a²⁺ or Cu_b²⁺), which are located in extraframework exchangeable positions and have different thermo-redox properties than Cu atoms in |Cu_5.7|[Si₂₃Al]-MCM-41.

It was verified that Cu atoms in |Cu_x|[Si_yAl]-MFI catalysts have greater specific activities in the conversion of NO to N₂ and in propane oxidation than copper atoms in |Cu_5.7|[Si₂₃Al]-MCM-41 and |CuO_0.15|[Si₁₁Al]-MFI. This behaviour confirms that Cu species, which are compensating the negative charge of the zeolite structure, irrespective of their nature (Cu_a²⁺, Cu_b²⁺), are responsible for the activity in both reactions.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the financial support received from Conselho Nacional de Desenvolvimento Científico e Tecnológico for this study (CNPq/Brazil, grants 477759/2003-3 and 551008/2002-4). Dr. M. S. Batista thanks Fundação de Amparo à Pesquisa do Estado de São Paulo, Brazil for his doctoral scholarship (FAPESP, grant 1998/02495-5). The support of Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG/Brazil, grant TEC–1241/01) is also acknowledged.

Received: January 12, 2003

Accepted: January 11, 2005

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*

To whom correspondence should be addressed

Publication Dates

• Publication in this collection
  28 Sept 2005
• Date of issue
  Sept 2005

History

• Accepted
  11 Jan 2005
• Received
  12 Jan 2003