Alkaline Treatment of Iron-Containing MFI Zeolites. Influence on

J. Phys. Chem. B 2006, 110, 20369-20378

20369

Alkaline Treatment of Iron-Containing MFI Zeolites. Influence on Mesoporosity Development and Iron Speciation Johan C. Groen,*,† Lluı´s Maldonado,‡ Elise Berrier,§ Angelika Bru1 ckner,§ Jacob A. Moulijn,† and Javier Pe´ rez-Ramı´rez*,‡,| DelftChemTech, Delft UniVersity of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands, Laboratory for Heterogeneous Catalysis, Institute of Chemical Research of Catalonia (ICIQ), AV. Paı¨sos Catalans 16, E-43007 Tarragona, Spain, Institute for Applied Chemistry Berlin-Adlershof, P. O. Box 961156, D-12474, Berlin, Germany, and Catalan Institution for Research and AdVanced Studies (ICREA), Passeig Lluı´s Companys 23, E 08010, Barcelona, Spain ReceiVed: July 10, 2006; In Final Form: August 15, 2006

The effects of alkaline treatment on the mesoporosity development and iron speciation in Fe-MFI zeolites have been investigated. To this end, a variety of samples derived from different synthetic routes and having distinct Si/Al ratios and Fe content were treated in NaOH solutions and characterized by N2 adsorption, SEM, TEM, UV/vis spectroscopy, and EPR. The alkaline treatment induces a significant intracrystalline mesoporosity development by framework silicon extraction and promotes disintegration of oligomeric iron species. Iron in framework positions has shown to provoke mesopore formation, whereas nonframework iron species suppresses silicon leaching and lowers the extent of extra porosity.

1. Introduction Zeolites are hydrothermally stable crystalline aluminosilicates with a well-defined network of pores of molecular dimensions and strong Brønsted acidity. This unique combination of properties confined in one material results in an abundant use of zeolites as catalysts in a variety of applications. The purely microporous nature, however, often affects the catalytic performance due to restricted mass transfer.1 Consequently, development of zeolites with a hierarchical architecture of porosity, i.e., a combination of micro- and mesopores in the same material, is increasingly attracting researchers’ interest. The intrinsic micropores serve as microreactors, and the larger pores facilitate physical transport of reactants and products to and from the active sites confined in these micropores. Various approaches exist to obtain hierarchically structured zeolites.2 Recently, we have reviewed the novel methodology of desilication as being a promising approach to achieve substantial mesoporosity development in zeolites, particularly in the case of ZSM-5.3 The mesoporosity is obtained by preferential framework silicon extraction upon treatment in alkaline medium, leading to a much higher degree of mesoporosity compared to dealumination as, e.g., obtained by steam treatment or acid leaching.4 Moreover, desilication hardly impacts on the framework aluminum species and connected acidic properties. This was furthermore confirmed in a recent study, where alkalinetreated ZSM-5 showed an enhanced capacity for subsequent ion exchange with iron due to the enhanced accessibility of the ionexchange positions.5 Framework silicon extraction has previously been used by other researchers as a methodology to alter * Corresponding authors. E-mail: [email protected] (J.C.G.); [email protected] (J.P.-R.). † Delft University of Technology. ‡ ICIQ. § Institute for Applied Chemistry Berlin-Adlershof. | ICREA.

microporous or ion-exchange properties.6-8 So far, however, desilication has not been applied to metal-containing zeolites. In this respect, besides its potential for mesoporosity formation, the desilication treatment could further impact on the distribution and/or nature of the resulting metal species. Iron-containing zeolites, particularly those derived from MFItype framework structures, have been intensively researched in the past decade as a result of their remarkable catalytic performance in a variety of applications. Well-known in this respect is the use of Fe-ZSM-5 in deNOx,9 direct N2O decomposition,10 and N2O-mediated benzene hydroxylation.11 The latter two processes are being commercialized by Uhde (EnviNOx process)12 and Solutia (AlphOx process),13 respectively. The synthesis method by which the iron is introduced in the microporous aluminosilicate matrix is crucial for the achievement of the required active species. Accordingly, the preparation procedure highly determines the performance of the obtained catalyst. Basically the following approaches can be discerned: (i) In liquid-ion exchange, an iron salt is dissolved in a suitable liquid, which is subsequently used to ion exchange the zeolite. (ii) In solid-ion exchange, a solid iron salt is mechanically mixed with the zeolite powder and afterward calcined in a furnace. (iii) With chemical vapor deposition, an iron salt susceptible to sublimation is heated in the presence of the acidic zeolite and the volatile iron species react with the exchangeable protons. (iv) In isomorphous substitution, iron is incorporated in framework positions during hydrothermal synthesis of the zeolite followed by calcination and steam treatment to create active nonframework iron species. Extraction of framework ions also induces defects in the zeolite framework, which in turn leads to (limited) mesoporosity. Despite the many encouraging studies on existing and emerging applications of iron-based zeolites, multiple examples are available in the literature where catalyst deactivation by coke

10.1021/jp0643193 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/23/2006

20370 J. Phys. Chem. B, Vol. 110, No. 41, 2006 formation has been encountered.14-17 Particularly in oxidative transformations of hydrocarbons over iron-containing zeolites, coking can dramatically impact the catalyst lifetime and overall performance, which should be attributed to the microporous nature of the zeolite-based catalyst. Also, here, improved transport in the microporous network has frequently been raised as a way to partially overcome this drawback, e.g., by a combination of micropores and mesopores in the same zeolite structure. The present work was undertaken to systematically investigate the influence of iron (both framework and nonframework species) on the mesoporosity development upon alkaline treatment of Fe-MFI zeolites. In addition, the ability of the alkaline treatment to induce changes in the iron speciation has been examined. To this end, various iron-containing MFI zeolites were prepared with different Si/Al ratios (15, 25, and >4000) using a variety of synthesis routes (liquid-ion exchange methods and isomorphous substitution followed by steam activation) and different iron contents (0-1.2 wt %). Characterization of the calcined and alkaline-treated materials was done by inductively coupled plasma optical emission spectroscopy (ICP-OES), N2 adsorption, SEM, TEM, UV/vis, and EPR. 2. Experimental Section 2.1. Preparation of Iron-Containing Zeolites. The commercial ZSM-5 zeolites used in this study were supplied by Zeolyst (CBV 3024E, NH4 form, sample code in this paper Z15) and Chemie Uetikon (PZ-2/40, Na form, sample code Z25) and have nominal molar Si/Al ratios of 15 and 25, respectively. Z25 was brought into the NH4 form via ion exchange in a 0.1 M NH4NO3 solution. Prior to further investigations, the samples were calcined in static air at 823 K for 5 h. The physicochemical properties are listed in Table 1. 2.1.1. Liquid-Ion Exchange. Iron Powder. The calcined zeolites Z15 and Z25 were ion exchanged using iron powder (Merck, particle size 10 µm) in the presence of hydrochloric acid at 323 K according to the method described by Schwidder et al.18 Approximately 6 g of the zeolite was stirred at 1000 rpm in an aqueous solution of concentrated HCl (6 cm3) in demineralized water (600 cm3) for 4 days under continuous flushing with argon to avoid oxidation from Fe2+ to Fe3+. The exchange was performed at a nominal iron concentration of 1.5 wt %. Iron Sulfate. Z15 and Z25 have also been exchanged with iron sulfate, FeSO4‚7H2O (Scharlau, extra pure), at a nominal charge of 1.5 wt % as described by Pieterse et al.14 To this end, ca. 3 g of calcined zeolite was stirred with 0.224 g of iron sulfate at 500 rpm and a temperature of 353 K in 600 cm3 aqueous solution under argon atmosphere for 6 h. Mohr Salt. Ion exchange with Mohr salt, (NH4)2Fe(SO4)2‚ 6H2O (Scharlau, extra pure), has been performed with Z25 at a nominal charge of 1.5 wt %, applying the same preparation conditions as for iron sulfate. After ion exchange, all catalysts were filtered, dried at 353 K, and subsequently calcined at 873 K: first the temperature was raised from room temperature to 423 K at 2 K min-1 and, after a 15 min isothermal period, the temperature ramp was continued with 5 K min-1 to 873 K, where the samples were left for 2 h. The subscripts “M”, “S”, and “MS” in the ionexchanged catalysts refer to metallic iron, iron sulfate, and Mohr salt as iron precursors, respectively. 2.1.2. Isomorphous Substitution and Steaming. Besides the ion-exchanged catalysts, isomorphously substituted iron silicalite (FeS) and iron ZSM-5 (FeZ) were synthesized hydrothermally

Groen et al. using TPAOH as template including iron nitrate in the synthesis gel.19 Upon calcination of the as-synthesized samples at 823 K for 5 h, the iron in the resulting catalyst was shown to be predominantly present in framework positions. The calcined samples were also steam-treated in a flow of steam and helium (300 mbar of steam and 30 cm3 min-1 He at 873 K for 5 h) to extract iron from the zeolite lattice into nonframework positions. The prefix “s-” denotes the steamed samples. 2.2. Alkaline Treatment. Approximately 1 g of zeolite sample was vigorously stirred in 30 cm3 of 0.2 M NaOH (Merck, extra pure) solution at 338 K for 30 min. Subsequently, the reaction was quenched by submersion of the flask in an icewater bath. The suspension was then filtered and washed with distilled water to achieve neutral pH. Finally, the zeolites were dried at 373 K and subsequently calcined in static air at 823 K for 5 h. The suffix “at” denotes the alkaline-treated samples. 2.3. Characterization. Chemical composition of the samples and the filtrates obtained upon alkaline treatment was determined by ICP-OES (Perkin-Elmer Optima 4300DV). N2 adsorption at 77 K was carried out in a Quantachrome Autosorb-6B apparatus. Samples were previously evacuated at 623 K for 16 h. The micropore volume (Vmicro) and the macroand mesopore surface area (Smeso) were determined with the t-plot method according to Lippens and de Boer.20 The BET method21 was applied to calculate the total surface area (SBET) of the samples, which is used for comparative purposes. Scanning electron microscopy (SEM) images were recorded at 5 kV in a JEOL JSM-6700F microscope. Samples were coated with gold to create contrast. Transmission electron microscopy (TEM) was carried out in a Zeiss 10 CA electron microscope operated at 100 kV. A few droplets of the sample suspended in chloroform were placed on a carbon-coated copper grid followed by evaporation at ambient conditions. UV/vis measurements were performed with a Cary 400 spectrometer (Varian) equipped with a diffuse reflectance accessory (Praying Mantis, Harrick). To reduce light absorption, samples were diluted with R-Al2O3 (calcined at 1473 K for 4 h) in a ratio of 1:3. Spectra were recorded after treatment in oxidative (pure O2, 20 cm3 min-1, 823 K) and reductive (20 vol % H2/Ar, 20 cm3 min-1, 823 K) conditions. The measured spectra were converted into Kubelka-Munk functions. EPR spectra in X-band (ν ≈ 9.5 GHz) were recorded at room temperature and 77 K with the CW-spectrometer ELEXSYS 500-10/12 (Bruker) using a microwave power of 6.3 mW, a modulation frequency of 100 kHz, and a modulation amplitude of 0.5 mT. The magnetic field was measured with respect to the standard 2,2-diphenyl-1-picrylhydrazyl hydrate (DPPH). For measurements at 77 K, a finger Dewar was used. 3. Results and Discussion 3.1. N2 Adsorption. 3.1.1. Ion-Exchanged Zeolites. N2 adsorption experiments over the calcined commercial zeolites Z15 and Z25 exemplify their predominantly microporous nature. The IUPAC type I isotherm with a low uptake at higher relative pressures confirms a relatively small contribution of mesoporosity (Figure 1). The parameters derived from the adsorption isotherms are summarized in Table 1. The micropore volumes of 0.15-0.16 cm3 g-1 and mesopore surface areas of 35-40 m2 g-1 confirm the similar nature of both zeolites. The relatively high framework aluminum concentration of the two commercial zeolites enables incorporation of a significant iron concentration in ion-exchange positions, while being in (Z25) or below (Z15) the optimal range of Si/Al ratios for mesopore formation by

Alkaline Treatment of Iron-Containing MFI Zeolites

J. Phys. Chem. B, Vol. 110, No. 41, 2006 20371

Figure 1. N2 adsorption isotherms at 77 K of calcined (a) Z15 and (b) Z25 and thereof derived ion-exchanged iron-containing zeolites. Open symbols denote adsorption; solid symbols denote desorption.

Figure 2. N2 adsorption isotherms at 77 K of alkaline-treated (a) Z15 and (b) Z25 and ion-exchanged iron-containing zeolites. Open symbols denote adsorption; solid symbols denote desorption.

TABLE 1: Textural Properties and Chemical Composition of the Zeolites and the Silicon Concentration in the Filtrates Obtained upon Alkaline Treatment sample

molar Si/Ala

Fe,a wt %

SBET,b m2 g-1

Vmicro,c cm3 g-1

Smeso,c m2 g -1

Z15 Z15at FeM-Z15 FeM-Z15at FeS-Z15 FeS-Z15at Z25 Z25at FeM-Z25 FeM-Z25at FeS-Z25 FeS-Z25at FeMS-Z25 FeMS-Z25at FeS FeSat s-FeS s-FeSat FeZ FeZat s-FeZ s-FeZat

16 15 16 16 16 16 26 19 26 21 26 21 26 20 >4000 >4000 >4000 >4000 33 27 35 32

0.02 0.02 0.74 0.76 0.78 0.81 0.02 0.02 1.1 1.3 0.5 0.65 0.3 0.36 1.09 1.20 1.05 1.25 0.6 0.7 0.6 0.65

415 390 395 375 410 410 405 500 410 415 410 420 405 425 425 415 425 420 410 415 355 385

0.15 0.14 0.15 0.13 0.15 0.13 0.16 0.13 0.16 0.13 0.16 0.11 0.16 0.11 0.17 0.13 0.17 0.13 0.16 0.13 0.16 0.14

40 50 50 50 40 45 35 195 40 115 40 140 40 165 15 135 20 125 20 125 40 80

a

b

[Si]filtrate,a mg L-1 570 290 290 3600 540 640 1210 4020 690 3000 349

c

ICP-OES. BET method. t-plot.

desilication.22 Introduction of iron via ion exchange of the different iron precursors does not significantly impact on the adsorption isotherms (Figure 1). Despite the different iron loadings, particularly in the Z25 series (0-1.1 wt % Fe), the coinciding isotherms point toward a preserved porous structure

and no substantial micropore blocking by the incorporated iron species. Alkaline treatment of the Z15 series leads to a very limited mesopore formation. Both the parent sample and the ionexchanged zeolites hardly exhibit an alteration in the adsorption

20372 J. Phys. Chem. B, Vol. 110, No. 41, 2006

Figure 3. Mesopore surface areas and concentration of Si in filtrate upon alkaline treatment of differently prepared Fe-Z25 samples. Black and gray bars denote mesopore surface areas of nontreated and alkalinetreated zeolites, respectively. The open bars represent silicon concentration in the filtrate.

isotherm (Figure 2). The low susceptibility of the Z15 series toward the alkaline treatment should be attributed to the relatively low framework Si/Al ratio in these samples. It has been previously shown that ZSM-5 zeolites with framework Si/ Al ratios 4000, the newly generated mesopore surface area of 120 m2 g-1 (135 m2 g-1 in the alkaline-treated sample - 15 m2 g-1 in the calcined zeolite) correlates well with the previously proposed dependency of the mesopore surface area on the framework Si/ Al ratio, as shown in Figure 5,22 provided that framework aluminum and iron are considered to be equivalent contributors. Apparently iron in framework positions impacts on the silicon extraction and connected mesopore formation to a similar extent as framework aluminum. This should then be attributed to its trivalent character (M3+) that analogous to aluminum induces a net negative charge in the zeolite framework, thereby suppressing extraction of the neighboring silicon atoms by OHions.23 Alkaline treatment of steamed iron silicalite leads to a slightly lower mesopore surface area and formation of somewhat larger pores and higher pore volume as compared to FeSat, as can be derived from the increased uptake in the N2 adsorption isotherm at higher relative pressures (Figure 4b). The formation of larger pores must be associated with the higher framework Si/Fe ratio in s-FeS than in FeS, as a result of the extraction of framework iron species in the former. Figure 4c shows that FeZat also presents a distinct mesoporosity development after alkaline treatment, leading to a mesopore surface area of 125 m2 g-1. Similar to iron silicalite, the Si/M3+ ratio of 27 in FeZ reasonably correlates with the Smeso vs Si/Al ratio relationship in Figure 5. The steam-treated s-FeZ sample reveals a relatively low susceptibility toward alkaline treatment; the mesopore surface area only increases from 40 to 80 m2 g-1. The significantly suppressed mesopore formation in s-FeZat is, besides the presence of nonframework iron species, mostly a result of extraframework aluminum species. Particularly the latter species have previously shown to negatively influence the mesopore formation efficiency by silicon extraction, due to realumination of these species during the alkaline treatment leading to an aluminum-rich outer surface.23 3.2. Chemical Composition. It has previously been shown by systematic analyses of the filtrates obtained upon alkaline treatment of a variety of ZSM-5 zeolites that the mesopore formation upon alkaline treatment is directly related to preferential framework silicon extraction, whereas the dislodgement of framework aluminum is generally several orders of magnitude lower.23 Accordingly, the framework Si/Al ratio in the resulting zeolite decreases upon alkaline treatment. Moreover, at an increased Si/Al ratio of the parent zeolite, alkaline treatment leads to a higher degree of silicon extraction and accordingly a higher concentration of silicon can be measured in the filtrate.



Figure 4. N2 adsorption isotherms at 77 K of (a) FeS, (b) s-FeS, (c) FeZ, and (d) s-FeZ and their alkaline-treated products. Open symbols denote adsorption; solid symbols denote desorption.

Figure 5. Evolution of mesopore surface area vs molar Si/Al ratio of alkaline-treated commercial MFI zeolites upon 30 min of alkaline treatment in 0.2 M NaOH at 338 K (after ref 22).

Indeed, the filtrate obtained from alkaline treatment of sample Z25 contains 3600 mg of Si L-1, while the one derived from Z15at only measures 570 mg of Si L-1 (Table 1). Interestingly, introduction of iron via ion exchange limits silicon dissolution to a larger extent than expected based on the obtained mesoporosity development as observed in the previous section. The mesopore surface area of 165 m2 g-1 obtained upon alkaline treatment of FeMSZ25 is only 15% lower than in Z25at, whereas the silicon concentration of 1210 mg of Si L-1 in the filtrate is 3 times lower (Figure 3). Likewise, the other iron-containing Z25 zeolites reveal limited dissolution of silicon to the filtrate and a higher concentration of iron leads to a lower degree of silicon leaching. In agreement, the 10-15 wt % weight loss upon alkaline treatment of the iron-containing Z25 zeolites is significantly lower than the 30-35 wt % obtained for the ironfree zeolites. Although less pronounced, the influence of iron on the silicon leaching proceeds analogously in the Z15 series. Even in the absence of distinct mesoporosity development, the presence of 0.7-0.8 wt % iron halves the silicon dissolution. Apparently, iron in nonframework positions interacts with the

extracted silicon species, partially avoiding its dissolution. This hypothesis is further supported by alkaline treatment of the isomorphously substituted zeolites. Desilication of FeS leads to a relatively high concentration of 4020 mg of Si L-1 in the filtrate, whereas alkaline treatment of its steamed counterpart (s-FeS) results in a similar mesoporosity development but markedly (6 times) lower silicon dissolution (690 mg of Si L-1). As only the latter sample contains nonframework iron species, these indeed should be held responsible for the limited silicon dissolution. The low silicon concentration in the filtrate of s-FeZat should, besides the presence of nonframework iron species, mostly be ascribed to the extraframework aluminum species that highly impact the overall silicon extraction and connected mesoporosity development as such.23 3.3. SEM. Investigations by scanning electron microscopy confirm the similar morphology of the commercial zeolites Z15 and Z25. The samples consist of particles in the lower micrometer and submicrometer range, which are constituted of aggregated nanometer-sized crystals. In agreement with previous observations on iron-free ZSM-5 zeolites treated under optimal conditions, the overall morphology hardly changes upon alkaline treatment.28 From SEM investigations on the ion-exchanged iron-containing zeolites, a slightly reduced size of the zeolite aggregates after treatment with NaOH solutions can be inferred (Figure 6). This could indicate that some particle deagglomeration occurs, caused by dissolution of silicon at the boundaries of the aggregated crystals.5 As a result, some additional intercrystalline surface area can be generated, certainly only minor with respect to the intracrystalline porosity resulting from extraction of framework silicon. Similar reasoning applies to the isomorphously substituted zeolites, which exhibit larger average crystal sizes and a lower degree of agglomeration, emphasizing even more the intracrystalline nature of the generated mesoporosity as evidenced by N2 adsorption. 3.4. TEM. Transmission electron microscopy provides supporting evidence on the presence of iron oxide and eventual mesoporosity development. The ion-exchanged iron zeolites


Groen et al.

Figure 6. SEM micrographs of calcined and alkaline-treated FeS-Z25.

Figure 7. TEM micrographs of calcined and alkaline-treated FeS-Z25 with emphasis on iron nanoparticles (top) and mesoporosity development (bottom).

exhibit a high concentration of nanoparticles covering the lower nanometer size range up to ca. 10 nm, confirming a significant clustering of iron (Figure 7, top). The preservation of these nanoparticles upon alkaline treatment is clearly revealed in, e.g., FeS-Z25at, which qualitatively indicates that these particles do not seem to be affected by subjection to NaOH solutions. A similar preservation of iron oxide nanoparticles could be observed in s-FeZ and s-FeZat, although the degree of clustering was less extensive as compared to the ion-exchanged samples. No nanoparticles could be detected in s-FeS due to the low degree of iron extraction.19 Additionally, low magnification TEM images confirm the generation of substantial mesoporosity upon alkaline treatment. Calcined FeS-Z25 exhibits no distinct signs of extra porosity, whereas the alkaline-treated sample resembles perforated crystals (Figure 7, bottom), indicating a high degree of newly created mesoporosity in agreement with N2 adsorption. Particularly the well-defined isomorphously substituted iron zeolites provide unquestionable evidence for intracrystalline mesoporosity formation upon alkaline treatment. Alkaline-treated FeZ presents circular mesopores in the range of 10-20 nm, whereas the preserved crystalline lattice can still be observed (Figure 8). As TEM does not allow further quantification of the iron nanoparticles and besides does not provide information on the presence of smaller iron species, a more detailed assessment of the intrapore iron species was accomplished by UV/vis and EPR spectroscopic investigations.

Figure 8. TEM micrograph of FeZat.

3.5. UV/Vis. Diffuse reflectance (DR) UV/vis spectroscopy is a convenient tool to achieve semiquantitative information on the presence of different Fe3+ species. As shown in Figure 9, UV/vis spectra of the FeS-Z15 and FeS-Z25 samples are predominantly featured by two broad, partially overlapping Fe3+ r O charge transfer (CT) bands around 240 and 290 nm. Assignment of these bands has been extensively discussed previously25,29 and can be summarized as follows: a sub-band around 240 nm arises from isolated Fe3+ sites in tetrahedral symmetry while single sites in higher coordination (five or six oxygen ligands) give rise to an additional sub-band around 290



Figure 9. UV/vis spectra of preoxidized nontreated and alkaline-treated FeS-Z15 (bottom line, gray) and FeS-Z25 (top line, black).

Figure 10. UV/vis spectra of FeS-Z15 and FeS-Z25 series upon calcination in O2 (top line, black) and reduction in H2 (bottom line, gray).

nm. Bands observed between 300 and 400 nm are assigned to oligomeric clusters, and large iron oxide particles are responsible for sample absorption in the visible (λ > 400 nm) range. Figure 9 depicts Fe3+ sites responsible for UV/vis absorbance in the preoxidized FeS-Z15 and FeS-Z25 samples and corresponding alkaline-treated materials. Preoxidation, obtained by a 1 h oxidative treatment (20 cm3 of O2 min-1) at 873 K, is assumed to affect all reachable Fe species and therefore provides information on the nature and amount of redox-active iron sites. Interestingly, despite the larger amount of iron in the FeS-Z15 series, the corresponding values of the Kubelka-Munk function F(R) in FeS-Z15 are significantly lower than those in the FeSZ25 series. This observation should be attributed to the presence of a larger degree of non-reoxidizable Fe2+ in the high-Alcontent material (FeS-Z15 series). Alkaline treatment induces pronounced changes in the iron speciation in both FeS-Z15 and FeS-Z25: tetrahedral isolated Fe3+ is promoted at the expense of higher coordinated species. This respeciation is clearly accompanied by a loss of reoxidizable iron in FeS-Z25at, whereas no distinct decrease is observed in FeS-Z15at. The significant mesoporosity development and connected requisite of framework silicon extraction upon alkaline treatment of FeSZ25 (Table 1), being much less pronounced in FeS-Z15, should likely be held responsible for this difference. Moreover, alkaline

treatment does not seem to affect clusters and particles in the FeS-Z25 series, in agreement with TEM investigations over these samples (Figure 7), whereas an enhanced clustering upon alkaline treatment of FeS-Z15 can be observed. The different degree of clustering upon alkaline treatment of the various zeolites might be related to the absence (FeS-Z15) or presence (FeS-Z25) of a substantial amount of extracted silicate species, which are able to accommodate and “disperse” the iron. Figure 10 shows the UV/vis spectra of the calcined and alkaline-treated FeS-Z15 and FeS-Z25 samples upon the following successive treatments: (i) calcination in O2 (1 h at 823 K) and (ii) reduction in H2/Ar mixture (1 h at 823 K). The nontreated catalysts exhibit a clear ability for reduction to Fe2+, as can be concluded from the distinct decrease in intensity of the absorption bands. On the other hand, the alkaline-treated samples appear to be less sensitive toward reductive treatment, most probably because of iron migration to framework positions and/or the formation of a separate iron silicate like phase, in which the iron is (partially) shielded by the silicon matrix. 3.6. EPR. X-band EPR spectra of the catalysts measured at 298 and 77 K show the characteristic signals at g-values of 2 and 4.3 and a shoulder at g′ ∼ 8.3, which are frequently observed for Fe species in zeolites and other oxide matrixes (Figure 11). It has been shown that the signal at g′ ∼ 4.3 coupled


Groen et al.

Figure 11. EPR spectra measured at room temperature (solid lines) and 77 K (dashed lines) of (a) FeS-Z25, (b) FeS-Z25at, (c) FeS-Z15, and (d) FeS-Z15at.

to the shoulder around g′ ∼ 9 arises from isolated Fe3+ sites in strong rhombic distortion.30 In zeolites, the line at g′ ∼ 4.3 has been equivocally assigned to Fe3+ sites in tetrahedral coordination, in either framework30-32 or extraframework positions.33-35 EPR signals at g′ ∼ 2 have been typically assigned to iron oxide clusters. However, highly symmetric isolated Fe3+ ions also contribute to isotropic lines at g′ ∼ 2 independent of their coordination number.36,37 The latter can be distinguished from the larger clusters by the temperature dependence of the g′ ∼ 2 signal intensity. For isolated Fe3+ sites, the signal intensity should increase upon cooling from room temperature to 77 K, since the magnetic susceptibility is inversely proportional to the temperature according to the Curie-Weiss law. Comparison of EPR spectra in Figure 11a,b shows a higher intensity of the g′ ) 4.3 signal in sample FeS-Z25at as compared to FeS-Z25, which correlates well with the higher percentage of isolated tetrahedral Fe3+ in the former, as evidenced by UV/vis spectrosopy in the previous section. Moreover, two signals of different line widths can be distinguished at g′ ) 2. Most probably, the broad subsignal arises from small oligonuclear Fe3+ clusters inside the pores. Large Fe2O3 particles visible in the TEM micrographs are antiferromagnetic in the applied temperature range and should not contribute to the EPR signal. The narrow subsignal is likely due to isolated Fe3+ in highly symmetric environment, which is supported by the fact that the narrow line at g′ ) 2 shows typical Curie-like behavior in both samples. In contrast, the intensity difference of the broad subsignal at 77 K and room temperature in sample FeS-Z25 (Figure 11a) is lower than expected for a pure paramagnetic species according to Curie’s law. This suggests that the respective Fe sites are coupled by weak antiferromagnetic interactions. After alkaline treatment, the intensity behavior of the broad line becomes more Curielike (Figure 11b), suggesting that the size of the small clusters might have decreased. This observation is also in good agreement with the UV/vis results that suggested partial disintegration of small clusters into isolated tetrahedral Fe3+. Samples FeS-Z15 and FeS-Z15at (Figure 11c,d) show similar EPR features. The changes upon alkaline treatment are, however, much less pronounced than in the FeS-Z25 series. This is related to the observation that Si extraction is much less pronounced upon alkaline treatment of FeS-Z15 and as a result

the nature of the Fe sites is less affected. The limited modification of the iron species constitution in the FeS-Z15 series is also in line with the UV/vis results. 3.7. Nonframework Iron Species and Alkaline Treatment: Formation of Iron Silicate Like Phase or Iron Reinsertion? The results in the previous sections have shown that framework and nonframework iron species influence differently the mesoporosity development upon alkaline treatment and even to a larger extent impact differently the leaching of silicon to the filtrate. Framework iron species act similarly to framework aluminum and provoke controlled mesoporosity development accompanied by substantial leaching of silicon to the filtrate (sample FeS). In contrast, nonframework iron species seem to restrict mesoporosity development and dramatically influence leaching of silicon (e.g., ion-exchanged FeZ15 and FeZ25 zeolites, see Figure 3). This is clearly corroborated by a systematic analysis of isomorphously substituted iron silicalite (FeS) and its steamed variant (s-FeS). Previous UV/vis experiments over calcined FeS have shown that its redox activity was rather poor due to the fact that iron is predominantly present in framework positions, and thus resistant to reduction.25 Upon steam treatment of FeS, which has shown to result in the formation of mostly isolated and oligonuclear iron sites, some reduction of iron could be obtained after exposure to H2 due to the existence of redox-active nonframework iron species. Interestingly, alkaline treatment of steamed iron silicalite (sFeSat) impedes the original redox activity of these nonframework iron species (Figure 12), while analysis of the filtrate derived from this sample has shown a strongly decreased silicon concentration (Table 1; 4020 mg of Si L-1 in FeSat vs 690 mg of Si L-1 in s-FeSat). This indicates that the iron that was extracted during steam treatment of FeS, i.e., isolated and oligonuclear sites, is responsible for the restricted silicon leaching more than iron oxide particles, as the latter are absent in s-FeS. In addition, the observation that these species have become redox inactive upon alkaline treatment suggests redox properties similar to those of iron in framework positions. Thus, nonframework iron species could have been (partially) reintegrated in the zeolite framework during treatment in NaOH solution, which in turn could explain the suppressed susceptibility of the ion-exchanged iron-containing Fe-Z25 zeolites toward the alkaline treatment (Figure 3). Integration of iron in the zeolite


J. Phys. Chem. B, Vol. 110, No. 41, 2006 20377 its adequacy to other zeolite types such as beta47 and ZSM12,48 the approach presented here should as well be applicable to other iron-containing zeolitic catalysts. We are currently investigating the possibility of combining posttreatments in order to simultaneously (a) create mesoporosity and (b) acquire the desired active iron species so as to modulate the catalyst constitution associated with its particular application. 4. Conclusions

Figure 12. UV/vis spectra of s-FeSat upon various treatments.

framework during alkaline treatment increases the framework iron content on the external surface where silicon has been extracted, thereby decreasing its Si/M3+ ratio, leading to a lower mesoporosity development (see Figure 5). Moreover, a higher concentration of nonframework iron species likely increases the chance of insertion in the framework during the alkaline treatment and thus can explain the lower degree of mesopore formation with increasing iron content (Figure 3). A similar though more pronounced impact of reinsertion of trivalent cations in the zeolite framework was previously observed upon treatment of steamed ZSM-5 zeolites in NaOH solutions. The extraframework aluminum species created by steam treatment exhibited realumination upon alkaline treatment and dramatically lowered the susceptibility of the material toward desilication.38 The insertion of iron into the zeolite lattice, however, cannot easily explain the dramatically decreased concentration of silicon in the filtrate obtained upon alkaline treatment and the significantly lower micropore volume (Table 1) in the presence of extraframework iron species. Accordingly, the possible formation of an iron silicate phase cannot be ruled out either. X-ray diffraction analyses of the alkaline-treated iron zeolites did not, however, reveal the existence of such a (amorphous) phase. Formation of such an additional solid phase requires silicon species that consequently will not be measured in the filtrate and could partially block the microporosity as measured in the alkaline-treated Fe-Z15 and Fe-Z25 samples. Moreover, if this iron silicate phase is present to a large extent and is presumed to have a low degree of porosity, it will “dilute” the zeolitic phase, leading to an overall lower porosity. Similar interaction of iron species with silicates was previously reported for waste treatment in which sodium silicates were added to capture iron and form an iron silicate precipitate39 and formation of iron silicate sediments in the Red Sea.40 At this stage it is not possible to provide conclusive evidence for each of the explanations and it is speculated that both phenomena occur in parallel. 3.8. Prospective Implications in Catalysis. It is widely accepted that the multiple reactions catalyzed by Fe-containing zeolites are structure sensitive.41-46 As a consequence, the altered iron distribution reported in this paper opens clear possibilities to impact on the catalytic performance of the posttreated samples. The promising potential of the heredescribed modified Fe-ZSM-5 catalysts can be envisaged in those conversions requiring isolated iron sites, due to the higher concentration of these species. Besides the altered redox properties and iron speciation, the created mesoporosity can be beneficial in providing improved access to the active sites. As desilication in alkaline medium, in view of framework silicon extraction and related mesoporosity development, has shown

Alkaline treatment of iron-containing MFI zeolites induces mesoporosity development and changes in iron speciation. Iron in framework positions provokes silicon extraction and connected mesopore formation upon alkaline treatment to a similar extent as framework aluminum. Solid correlations between isomorphously substituted Fe-MFI samples and iron-poor ZSM-5 zeolites in view of framework silicon extraction and connected mesoporosity development have been established. Alkaline treatment of Fe-ZSM-5 zeolites with iron in nonframework positions systematically leads to a lower mesoporosity as compared to the iron-poor zeolites. The presence of nonframework iron furthermore dramatically impacts the leaching of silicon to solution and lowers the micropore volume in the resulting zeolites. UV/vis and EPR investigations have shown that the alkaline treatment furthermore promotes the disintegration of oligonuclear iron clusters into lower coordinated species, particularly at higher degrees of silicon extraction. References and Notes (1) Ka¨rger, J.; Ruthven, D. M. Diffusion in Zeolites and Other Microporous Materials; Wiley & Sons: New York, 1992. (2) Hartmann, M. Angew. Chem., Int. Ed. 2004, 43, 5880. (3) Groen, J. C.; Moulijn, J. A.; Pe´rez-Ramı´rez, J. J. Mater. Chem. 2006, 16, 2121. (4) Groen, J. C.; Moulijn, J. A.; Pe´rez-Ramı´rez, J. Microporous Mesoporous Mater. 2005, 87, 153. (5) Meliań-Cabrera, I.; Espinoza, S.; Groen, J. C.; van der Linden, B.; Kapteijn, F.; Moulijn, J. A. J. Catal. 2006, 238, 250. (6) Le Van Mao, R.; Xiao, S.; Ramsaran, A.; Yao, J. J. Mater. Chem. 1994, 4, 605. (7) Ohayon, D.; Le Van Mao, R.; Ciaravino, D.; Hzael, H.; Cochennec, A.; Rolland, N. Appl. Catal., A 2001, 217, 241. (8) Le Van Mao, R.; Ramsaran, A.; Xiao, S.; Yao, J.; Semmer, V. J. Mater. Chem. 1995, 5, 533. (9) Chen, H. Y.; Sachtler, W. M. H. Catal. Today 1998, 42, 73. (10) Pe´rez-Ramı´rez, J.; Kapteijn, F.; Mul, G.; Moulijn, J. A. Chem. Commun. 2001, 693. (11) Panov, G. I.; Uriarte, A. K.; Rodkin, M. A.; Sobolev, V. I. Catal. Today 1998, 41, 365. (12) Schwefer, M.; Szoll, E.; Turek, T. WO 01/51181, 2001. (13) Parmon, V. N.; Panov, G. I.; Uriarte, A.; Noskov, A. S. Catal. Today 2005, 100, 115. (14) Pieterse, J. A. Z.; Booneveld, S.; van den Brink, R. W. Appl. Catal., B 2004, 51, 215. (15) Pe´rez-Ramı´rez, J.; Gallardo-Llamas, A. J. Phys. Chem. B 2005, 109, 20529. (16) Selli, E.; Rossetti, I.; Meloni, D.; Sini, F.; Forni, L. Appl. Catal., A 2004, 262, 131. (17) Kollmer, F.; Hausmall, H.; Hoelderich, W. F. In Recent AdVances in the Science and Technology of Zeolites and Related Materials; van Steen, E., Callanan, L. H., Claeys, M., Eds.; Studies in Surface Science and Catalysis 154C; Elsevier: Amsterdam, 2004; pp 2688-2695. (18) Schwidder, M.; Santosh Kumar, M.; Klementiev, K.; Pohl, M. M.; Bru¨ckner, A.; Grunert, W. J. Catal. 2005, 231, 314. (19) Pe´rez-Ramı´rez, J.; Kapteijn, F.; Groen, J. C.; Domeńech, A.; Mul, G.; Moulijn, J. A. J. Catal. 2003, 214, 33. (20) Lippens, B. C.; de Boer, J. H. J. Catal. 1965, 4, 319. (21) Brunauer, S.; Emmet, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309. (22) Groen, J. C.; Jansen, J. C.; Moulijn, J. A.; Pe´rez-Ramı´rez, J. J. Phys. Chem. B 2004, 108, 13062. (23) Groen, J. C.; Peffer, L. A. A.; Moulijn, J. A.; Pe´rez-Ramı´rez, J. Chem.sEur. J. 2005, 11, 4983. (24) Camblor, M. A.; Corma, A.; Valencia, S. Microporous Mesoporous Mater. 1998, 25, 59.

20378 J. Phys. Chem. B, Vol. 110, No. 41, 2006 (25) Pe´rez-Ramı´rez, J.; Santosh Kumar, M.; Bru¨ckner, A. J. Catal. 2004, 223, 13. (26) Pe´rez-Ramı´rez, J.; Mul, G.; Kapteijn, F.; Moulijn, J. A.; Overweg, A. R.; Domeńech, A.; Ribera, A.; Arends, I. W. E. C. J. Catal. 2002, 207, 113. (27) Groen, J. C.; Peffer, L. A. A.; Moulijn, J. A.; Pe´rez-Ramı´rez, J. In Nanoporous Materials IV; Sayari, A., Jaroniec, M., Eds.; Studies in Surface Science and Catalysis 156; Elsevier: Amsterdam, 2005; pp 401-408. (28) Groen, J. C.; Peffer, L. A. A.; Moulijn, J. A.; Pe´rez-Ramı´rez, J. Microporous Mesoporous Mater. 2004, 69, 29. (29) Bordiga, S.; Buzzoni, R.; Geobaldo, F.; Lamberti, C.; Giamello, E.; Zecchina, A.; Leofanti, G.; Petrini, G.; Tozzolo, G.; Vlaic, G. J. Catal. 1996, 158, 486. (30) Aasa, R. J. Chem. Phys. 1970, 52, 3919. (31) Joshi, P. N.; Awate, S. V.; Shiralkar, V. P. J. Phys. Chem. 1993, 97, 9749. (32) Inui, T.; Nagata, H.; Takeguchi, T.; Iwamoto, S.; Matsuda, H.; Inoue, M. J. Catal. 1993, 139, 482. (33) Goldfarb, D.; Bernardo, M.; Strohmaier, K. G.; Vaughan, D. E. W.; Thomann, H. J. Am. Chem. Soc. 1994, 116, 6344. (34) Kucherov, A. V.; Slinkin, A. A. Zeolites 1988, 8, 110. (35) Wichterlova´, B.; Jiru, P. React. Kinet. Catal. Lett. 1980, 13, 197. (36) Bru¨ckner, A.; Lohse, U.; Mehner, H. Microporous Mesoporous Mater. 1998, 20, 207.

Groen et al. (37) Catana, G.; Pelgrims, J.; Schoonheydt, R. A. Zeolites 1995, 15, 475. (38) Groen, J. C.; Moulijn, J. A.; Pe´rez-Ramı´rez, J. Microporous Mesoporous Mater. 2005, 87, 153. (39) Bortoluzzi, R. L. Braz. Pedido PI, BR 2002002597, 2003. (40) Taitel-Goldman, N.; Singer, A. Clay Miner. 2002, 37, 235. (41) El-Malki, El-M.; van Santen, R. A.; Sachtler, W. M. H. J. Catal. 2000, 196, 212. (42) Kiwi-Minsker, L.; Bulushev, D. A.; Renken, A. J. Catal. 2003, 219, 273. (43) Pe´rez-Ramı´rez, J.; Bru¨ckner, A.; Kapteijn, F. J. Catal. 2003, 218, 234. (44) Pe´rez-Ramı´rez, J.; Kumar, M. S.; Bru¨ckner, A. J. Catal. 2004, 223, 13. (45) Heinrich, F.; Schmidt, C.; Lo¨ffer, E.; Menzel, M.; Gru¨nert, W. J. Catal. 2002, 212, 157. (46) Nobukawa, T.; Yoshida, M.; Kameoka, S.; Ito, S.; Tomishige, K.; Kunimori, K. J. Phys. Chem. B 2004, 108, 4071. (47) Groen, J. C.; Peffer, L. A. A.; Moulijn, J. A.; Pe´rez-Ramı´rez, J. Microporous Mesoporous Mater. 2004, 69, 29. (48) Wei, X.; Smirniotis, P. Presented at the AIChE 2005 Annual Meeting, Cincinnati, October 2005.

Alkaline Treatment of Iron-Containing MFI Zeolites. Influence on

Recommend Documents