Cation Migration and Structural Modification of Co-Exchanged

The in situ time-resolved analysis of Co-exchanged synthetic ferrierite was performed in the range 53−810 °C by Rietveld refinements of powder diff...
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J. Phys. Chem. B 2003, 107, 12973-12980

12973

Cation Migration and Structural Modification of Co-Exchanged Ferrierite upon Heating: a Time-Resolved X-ray Powder Diffraction Study Maria C. Dalconi,* Alberto Alberti, and Giuseppe Cruciani Department of Earth Sciences, Section of Mineralogy, UniVersity of Ferrara, C.so Ercole I° d’Este, 32 I-44100 Ferrara, Italy ReceiVed: March 19, 2003; In Final Form: September 16, 2003

The in situ time-resolved analysis of Co-exchanged synthetic ferrierite was performed in the range 53-810 °C by Rietveld refinements of powder diffraction data. The small contraction of the unit cell volume (-2.35%) confirmed that the ferrierite framework behaves as a noncollapsible framework. Moreover, continuous monitoring of the structural modifications induced by heating showed that cobalt ions migrate to new positions following the dehydration process step by step. Above 500 °C, five cation sites were localized; four of these are readily accessible to absorbed molecules and act as Lewis acid sites.

Introduction The removal of nitrogen oxides from the exhaust emitted by lean burn engines is nowadays a pressing necessity. The evergrowing research dedicated to zeolite-based materials exchanged with transition metals arise from the fact that transition metals in zeolite act as Lewis acid sites. Such exchanged zeolites exhibit high activity for the selective catalytic reduction (SCR) of nitrogen oxides (NOx), with hydrocarbons as reducing agents in the presence of oxygen. Co-exchanged ferrierites proved to be the most promising catalyst for reducing NOx with methane in the presence of excess oxygen in the temperature range 350500 °C1. The use of methane (readily available and widely used as a fuel) for the SCR of NOx is a greatly attractive approach for certain emission sources. To understand the mechanisms that confer high catalytic activity to the zeolites exchanged with transition metals, one needs to consider several points: (i) the local structure of the transition metal ion sites, particularly their accessibility; (ii) the interaction of ion sites with the zeolite framework and reactants (its coordination chemistry); and (iii) specific properties of the surrounding framework, such as its shape-selective properties.2 Given the wide range of the abovementioned points, it is necessary to apply different experimental techniques to provide complementary information, since each technique typically probes only one aspect of the complex system represented by zeolitic catalysts. The Co-ferrierite catalyst and its interaction with nitrogen oxides, CH4, and SO2 has been studied by several spectroscopic techniques (FTIR, UV-vis spectroscopy, EXAFS, temperature programmed desorption, X-ray photoelectron spectroscopy, magnetic susceptibility measurements),3-6 whereas the distribution of extraframework cobalt ions in dehydrated Co-exchanged ferrierite has been studied by FTIR and UV-vis techniques.3 On the basis of this research, three different Co2+ ion sites have been proposed, denoted as R-, β-, and γ sites. The β site is located at the center of the six ring separating two ferrierite cages (site G according to the notation of Mortier7), the R site is coordinated to the walls of the 10-ring channels (site B of Mortier), while the γ site is located inside the ferrierite cage, in the so-called “boatshaped” position (site C of Mortier) (Figure 1). The Si/Al distribution in ferrierite was studied by Fripiat et al.8 by ab initio * Corresponding author. E-mail: [email protected].

Figure 1. Suggested cationic sites of the R-, β-, and γ-type Co ions in FER (cf. refs 3 and 4).

molecular orbital calculations, whereas the stability of cobalt in the B and G sites was investigated by McMillan et al.9 by quantum chemical calculations utilizing the density functional theory (DFT) formalism. Sˇ poner et al.,10 using a combined experimental and theoretical approach, investigated the effect of Co2+ coordination on the charge distribution over the β cationic site. Despite this variety of approaches, diffraction techniques have never been used so far to locate extraframework cobalt ions in ferrierite. This study is devoted to continuous monitoring of structural modification and the migration of cobalt ions upon heating a Co-exchanged ferrierite by in situ synchrotron X-ray powder diffraction. This technique aims to provide useful information for a better comprehension of the catalytic reactions occurring at variably high temperatures in zeolites. Experimental Section Synthesis and Ion-Exchanges. The synthetic ferrierite Engelhard EZ-500 (K2.7Na1.1Al3.8Si32.2O72‚12H2O) was ion-

10.1021/jp030351a CCC: $25.00 © 2003 American Chemical Society Published on Web 11/01/2003

12974 J. Phys. Chem. B, Vol. 107, No. 47, 2003 exchanged to ammonium form by exhaustive exchange with aqueous solutions of ammonium nitrate. The Co-ferrierite sample was prepared by exchanging the ammonium form of ferrierite with Co-acetate aqueous solution, then calcined at 550° for 2 h, and rehydrated at room conditions. The cation content after exchange was determined by atomic absorption spectroscopy (AAS) and the water content was determined by thermogravimetric analysis; the resulting chemical formula was Co1.8Na0.2Al3.8Si32.2O72‚18H2O, with a Co/Al molar ratio about 0.5 corresponding to an almost complete Co2+ ion exchange. The crystal structure determination of the above synthetic ferrierite exchanged with different amounts of cobalt11 showed that this metal can almost completely substitute the K and Na cations of the as-synthesized ferrierite, but it does not form metallic clusters in the channel system of this zeolite. The sample used in this study (which is the sample Co-FER3S studied by Dalconi et al.11) is therefore characterized by the almost maximum fraction of Co2+ cations exchangeable in a ferrierite material with a Si/Al ratio equal to 8.5. This sample will be indicated as Co-FER from now on. In Situ X-ray Data Collection. In situ time-resolved X-ray powder diffraction, exploiting high-intensity synchrotron radiation combined with position-sensitive detector, is a powerful method for obtaining information about real-time structural modifications undergone by zeolites upon heating treatment. Temperature-resolved powder diffraction data of Co-exchanged ferrierite were collected at the GILDA beamline12 at the European Synchrotron Radiation Facility (ESRF), Grenoble (France), using a Debye-Scherrer geometry with an angledispersed set up13 based on a 2D imaging plate (IP) detector (200 × 400 mm2). The powder sample was packed in a 0.3mm capillary, open at both ends, and heated in situ up to 810 °C at a heating rate of 4.7 °C/min, using a hot air stream controlled by a Eurotherm controller. During heating, the twodimensional diffraction patterns were recorded on the 4-mm slitdelimited portion of the translating IP in the temperature range 53-810 °C. The capillary containing the sample powder was mounted horizontally on a goniometer head and kept rotating during acquisition to improve the grain statistics. The incident beam wavelength was chosen equal to 0.95337 Å, and the IP detector was positioned perpendicular to the sample at a distance of 205 mm. The parameters of the instrumental set up (sample to detector distance, tilt of the detector, instrumental peak broadening) were calibrated with a powder diffraction pattern of the calibration standard LaB6 using the FIT2D program.14 The powder diffraction patterns relative to the temperatureresolved acquisition were extracted from the two-dimensional image, integrating on strips corresponding to 10 °C and with an integration step of 20 °C. Data Analysis. The topological symmetry of ferrierite is orthorhombic, space group Immm.15 However, deviations from this symmetry are well known and depend on both extraframework or framework content. In natural Mg-rich ferrierite, although the Immm space group was successfully used in structure refinement, the real symmetry was consistent with the subgroup Pnnm;16 on the contrary, the natural Mg-poor, Na-, K-rich ferrierite crystallizes in the monoclinic P21/n symmetry,17 as well as the synthetic K, Na form18 used as a precursor of the Co-exchanged material used in this work, whereas the all-silica synthetic ferrierite displays an orthorhombic Pmnn19 symmetry. In the other structure refinements of hydrated or dehydrated ferrierite, the topological symmetry Immm18,20-23 was always assumed, even when there was evidence that the real symmetry was lower, but strongly pseudo-symmetric Immm.

Dalconi et al.

Figure 2. Plot of the relative values of the unit-cell parameters vs temperature. a0, b0, c0, and V0 refer to the refined unit cell parameters and volume of Co-FER at 53 °C.

Close inspection of the diffraction patterns of our sample did not reveal the presence of any reflections of the type h + k + l ) 2n + 1, which are forbidden in the Immm symmetry at every step of its dehydration process; the orthorhombic topological symmetry Immm was therefore adopted in all refinements in the temperature range 53-810°C. Moreover, there was no evidence for a lowering of symmetry as a consequence of the thermal treatment. The extracted patterns were analyzed using the Rietveld structure refinement approach as implemented in the GSAS package.24 The background curves were fitted with a shifted-Chebishev polynomial using 16 coefficients, and the diffraction profiles were modeled by a pseudo-Voigt peak shape function25 as implemented in GSAS. Only the histogram scale factor, background function coefficients, and unit cell parameters were varied in the early cycles of the refinement procedure. The profile function coefficients, atomic coordinates, and occupancy fractions were then optimized. In the structure refinement of Co-FER at 53 °C, the atomic displacement parameters (ADPs) of all the framework atoms were varied independently, whereas at higher temperatures the ADPs of all tetrahedral atoms, as well as those of all the framework oxygens, were constrained in order to be equal. Soft constraints were applied to the T-O framework bond length at the initial stages of the refinements. The Si/Al ratio ()8.4) resulting from chemical analysis was imposed on all framework cation sites. The crystal structure of NH4-exchanged ferrierite18 was adopted as starting structural model for Rietveld refinements of CoFER at 53 °C. The structure model of each increasing temperature step was taken from the previous refinement. Results and Discussion Ferrierite (FER) is a natural as well as a synthetic mediumpore zeolite of the mordenite family, whose crystal structure is based on five-ring building units linked in complex chains parallel to the [001] direction. The framework structure contains channels parallel to [001] with 10-ring openings, intersecting channels parallel to [010] with apertures formed by 8-ring openings. The 10-ring channel alternates along the b axis with the so-called “ferrierite cage”, namely, a [82626458] cage. Unit Cell Variations on Dehydration. The Co-exchanged ferrierite (Co-FER) heated to 810 °C behaves as a noncollapsible framework.26 At this temperature, the contraction of the cell volume is only 2.35%, mainly because of the decrease of parameter a. Figure 2 clearly shows that, whereas the decrease of unit cell parameters a and c mainly occurs below 400 °C, b suddenly changes its slope above this temperature. As a result, while about 80% of the decrease of a and c (which corresponds to a volume contraction of about 80% of the total) occurs below

In Situ Analysis of Co-Exchanged Ferrierite

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TABLE 1: Crystallographic Data and Experimental and Refinement Details for Co-FER at Different Temperatures sample

Co-FER 53 °C

Co-FER 305 °C

Co-FER 810 °C

formula space group a (Å) b (Å) c (Å) cell volume (Å3) ∆V (%) λ (Å) min/max 2θ (°) Rwp (%)b Rp (%)a Red-χ2d RF2 (%)e Nvar Nobs

Na0.2Co1.8Al3.8S i32.2O72‚18H2O Immm 19.061(1) 14.163(1) 7.493(1) 2022.8(1)

Na0.2Co1.8Al3.8 Si32.2O72‚4H2O Immm 18.878(1) 14.136(1) 7.476(1) 1995.0(1) -1.37 0.95337 4/57 3.6 2.7 15.4 4.9 60 5178

Na0.2Co1.8Al3.8Si32.2O72 Immm 18.819(1) 14.054(1) 7.467(1) 1974.9(1) -2.35 0.95337 4/57 3.3 2.5 7.9 5.7 48 5174

0.95337 4/57 3.5 2.8 11.0 4.5 65 5315

2 0.5 c 2 2 2 a R ) Σ[Y - Y ]/ΣY . b R 2 d 2 2 e p io ic io wp ) [Σwi(Yio - Yic) /ΣwiYio ] . λ ) 0.95337 Å. Red-χ ) Σwi(Yio - Yic) /(Nobs - Nvar). RF ) Σ|Fo - Fc |/ Σ|Fo2|.

Figure 3. Observed (crosses), calculated (solid line) diffraction patterns and difference curve from Rietveld refinements of Co-FER at 53 °C (a), Co-FER at 305 °C (b), and Co-FER at 810 °C (c).

400 °C, more than 60% of the decrease of b occurs above this temperature. It is not surprising that the smallest shortening is registered along the crystallographic direction c (-0.36%), which is parallel to the five-ring chains of ferrierite. Table 1 reports details of refinements and unit cell parameters for CoFER at 53 and 810 °Cswhich are, respectively, the lowest and almost the highest limits of the temperature rampsand 305 °C, an intermediate temperature of particular interest in the evolution of the ferrierite structure, while their Rietveld profile fittings are shown in Figure 3. Because of the difficulty in accurately calibrating the experimental parameters of the off-line imaging

plate setup, the reported unit cell parameters could differ significantly from their true values; on the other hand, their relative variations during the dehydration process (which are the most significant data for our study) are highly reliable. Extraframework Cobalt Ion Migration. The structure refinement of Co-FER at 53 °C indicates the presence of three extraframework Co2+ cation sites and six water molecule sites. The most populated cation site (Co1 in Table 2) is at the center of the ferrierite cage. Its coordination shows strong similarities with the coordination of both nickel in Ni-exchanged ferrierite and magnesium in natural Mg-rich ferrierite. Like these cations, Co2+ is coordinated to six water molecules in octahedral arrangement, with two possible configurations which differ by a rotation of about 45° around the z axis. However, whereas in natural ferrierite the center of the cage is fully occupied, in Co-, as well as in Ni-ferrierite, this site is only half occupied. This different behavior has been exhaustively discussed by Dalconi et al.,23 in their study on the crystal structure of Ni2+ exchanged ferrierite. A second cation site (Co2 in Table 2) is located near the center of the 10-ring channel (like Ni2 site in Ni-ferrierite) and is bonded with one W6 and two W5 water molecules in an almost regular triangular coordination, or with four W5 water molecules in an orthorhombic pyramidal coordination. A higher coordination number is prevented by short distances between water molecules W5 and W6. This site is near to the one occupied by K in natural Mg-rich hydrated ferrierite.16 The third cation site (Co3 in Table 2), which corresponds to the Ni3 site in Ni-ferrierite, is located in the 10-ring channel in front of the center of the 8-ring window separating the ferrierite cage from the 10-ring channel and coordinates one W3 and two W5 molecules; a fourfold coordination is hindered by the short W3-W3 distance. Therefore, none of these three cation sites is bonded to framework oxygens. The three Co polyhedra share one of their vertexes with the adjacent ones to form a chain Co1-W3-Co3-W5-Co2-W5-Co3-W3-Co1 (Figure 4). On the whole, about 1.7 Co cations and 16.4 water molecules were localized, in good agreement with the chemical analysis (see Table 1). The crystal structure of Co-FER at 53 °C strongly resembles that of the same sample at room temperature obtained by highresolution synchrotron data,11 both in coordinates and occupancies of the atoms, thus showing that no remarkable modification occurs in this range of temperature and confirming the reliability of the in situ data analysis. However, the temperature scale of our study should not be considered an absolute scale, as it clearly

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Dalconi et al.

TABLE 2: Atomic Coordinates, Fractions, Uiso Values, and Selected Interatomic Distances (Å) in Co-FER at 53 °C site

x/a

y/b

z/c

fractions

Uiso (×100 Å2)

T1 T2 T3 T4 O1 O2 O3 O4 O5 O6 O7 O8 Co1 Co2 Co3 W1 W2 W3 W4 W5 W6 T1-O3 ) 1.651(7)[x2] T1-O4 ) 1.602(6) [x2] T2-O1 ) 1.568(3) T2-O3 ) 1.620(7) T2-O7 ) 1.646(4) [x2]

0.1561(3) 0.0820(2) 0.2728(2) 0.3237(1) 0 0.2429(4) 0.1016 (3) 0.2012(3) 0.25 0.1585(3) 0.1145(2) 0.3222(2) 0 0.4517(14) 0 0 0.0929(7) 0.0508(7) 0 0 0.0572(8) T3-O2 ) 1.651(4) T3-O4 ) 1.605(6) T3-O8 ) 1.603(4) [x2] T4-O5 ) 1.597(2) T4-O6 ) 1.613(2) T4-O7 ) 1.591(4) T4-O8 ) 1.629(4)

0 0.2020(3) 0 0.2014(2) 0.2113(6) 0 0.0906(5) 0 0.25 0.2788(4) 0.2528(3) 0.0887(3) 0 0 0.2885(14) 0 0.0727(9) 0.1553(9) 0.4524(14) 0.6043(12) 0.5 Co1-W1 ) 2.15(1) [x2] Co1-W2 ) 2.05(1) [x4] Co1-W3 ) 2.40(1) [x4] Co2-W5 ) 2.11(1) [x4] Co2-W6 ) 1.68(2) [x2] Co3-W3 ) 2.12(2) [x2] Co3-W5 ) 1.93(2) [x2]

0 0 0.2931(4) 0.2072(3) 0 0.5 0 0.1804(8) 0.25 0.5 0.1793(5) 0.2505(5) 0.5 0 0.5 0.2128(16) 0.5 0.5 0.1131(17) 0.3406(22) 0.2772(20)

1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 0.474(3) 0.073(2) 0.108(2) 0.545(5) 0.371(5) 0.362(5) 0.318(3) 0.335(6) 0.371(5)

5.6(2) 6.4(1) 5.3(1) 5.5(1) 6.4(4) 6.7(4) 7.3(2) 6.6(2) 7.7(3) 5.0(2) 8.1(2) 7.6(2) 7.5(2) 10.0(2) 10.2(9) 9.9(6) 11.5(8) 11.8(8) 14.4(9) 13.5(9) 9.7(7)

Figure 5. Location of cobalt ion sites in the ferrierite cage at high temperature. The Co1 site relative to Co-FER at 53 °C is also reported for comparison.

Figure 4. Schematic representation of cobalt and water molecules site location as obtained from Rietveld refinements of Co-FER at low temperature.

depends on the experimental conditions such as particle size, sample preparation, heating rate, and H2O pressure. As the temperature increases, starting from about 100 °C the water molecules W1 and W2 tend to be progressively lost. At the same time, a fraction of the cations in the Co1 site, initially located at the center of the ferrierite cage, moves toward the center of the six-ring of the cage (but out of the plane of the oxygens by about 0.5 Å) to a new site called Co1a. As a consequence of the presence of Co2+ ions in site Co1a, the O1 framework oxygens shift toward the center of the six-ring window by about 0.6 Å and consequently toward the Co1a site (O1bis oxygen site in Tables 3 and 4). As a result the six-ring window of the ferrierite cage is deformed when Co atoms are

in Co1a, even if the ferrierite framework does not generally evidence any remarkable structural variations as a consequence of local distortion of the six-ring window. Around 200 °C, the majority of the water molecules in site W3 are also lost, and some of the residual cationssstill located in Co1 sitesmigrate to a new site (Co1b) near the walls of the ferrierite cage in a position usually known as “boat-shaped” position.3 Around 300 °C, water molecule sites W1 and W2 are completely empty, whereas a residual amount of H2O (around 10% occupancy) occupies W3. At the same time, about 90% of the Co atoms located in site Co1 at room temperature are now in site Co1a (about 0.5 atoms per unit cell) or in Co1b (about 0.2 atoms per unit cell). Above 400 °C, sites Co1 and W3 are empty too, and the Co cations are distributed over Co1a (about 0.75 atoms per unit cell) and Co1b (about 0.25 atoms). Site Co1a is therefore six-coordinated to the oxygens of the six-ring, that is, to four O3 and to two O1bis, whereas Co1b is threefold coordinated with one O6 and two O5 atoms (Figure 5). Sites Co1a and Co1b correspond to sites G and C in the notation of Mortier.7 These

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TABLE 3: Atomic Coordinates, Fractions, UIso Values, and Selected Interatomic Distances (Å) in Co-FER at 305 °C site

x/a

y/b

T1 T2 T3 T4 O1 O2 O3 O4 O5 O6 O7 O8 O1bis Co1 Co1a Co1b Co2 Co3 Co3a W3 W4 W5 W6

0.1579(3) 0.0847(2) 0.2730(2) 0.3243(1) 0 0.2488(5) 0.1044(4) 0.2008(4) 0.25 0.1520(3) 0.1125(3) 0.3213(2) 0 0 0 0.255(5) 0.421(6) 0 0 0.062(2) 0 0 0.064(1)

0 0.2004(3) 0 0.2022(2) 0.2157(8) 0 0.0895(5) 0 0.25 0.2805(4) 0.2531(3) 0.0905(3) 0.156(6) 0 0 0.245(7) 0 0.249(4) 0.289(2) 0.147(2) 0.447(2) 0.637(6) 0.5

T1-O3 ) 1.619(7) [x2] T1-O4 ) 1.544(7) [x2] T2-O1 ) 1.613(4) T2-O3 ) 1.611(7) T2-O7 ) 1.639(4) [x2] T2-O1bis ) 1.718(4)

T3-O2 ) 1.634(4) T3-O4 ) 1.609(7) T3-O8 ) 1.601(4) [x2] T4-O5 ) 1.593(2) T4-O6 ) 1.614(3) T4-O7 ) 1.593(4) T4-O8 ) 1.614(4)

Co1-W3 ) 2.39(1) [x4] Co1a-O3 ) 2.38(1) [ x4] Co1a-O1bis ) 2.25(8) [x2] Co1b-O5 ) 1.88(1) [x2] Co1b-O6 ) 1.92(1) Co2-W4 ) 2.40(6) [x4] Co2-W5 ) 2.52(6) [x4] Co3-W3 ) 1.85(5) [x2] Co3-W5 ) 1.73(6) [x2] Co3a-O1 ) 2.24(3) Co3a-O7 ) 2.27(1) [x2]

a

z/c

fractions

Uiso (100 Å2)a

0 0 0.2901(4) 0.2048(3) 0 0.5 0 0.1759(9) 0.25 0.5 0.1822(6) 0.2490(6) 0 0.5 0.060(2) 0.5 0 0.5 0.266(5) 0.5 0.268(6) 0.418(9) 0.128(4)

1.000 1.000 1.000 1.000 0.900 1.000 1.000 1.000 1.000 1.000 1.000 1.000 0.100 0.061(3) 0.112(1) 0.021(1) 0.025(2) 0.068(2) 0.055(2) 0.144(6) 0.112(5) 0.065(5) 0.145(5)

5.32(4)a 5.32(4)a 5.32(4)a 5.32(4)a 7.63(8)a 7.63(8)a 7.63(8)a 7.63(8)a 7.63(8)a 7.63(8)a 7.63(8)a 7.63(8)a 7.63(8)a 11(2) 8.7(8) 14(4) 13(5) 12(1) 12(1) 13(2) 15(3) 12(4) 15(2)

Uiso coupled during the refinement.

TABLE 4: Atomic Coordinates, Fractions, UIso Values, and Selected Interatomic Distances (Å) in Co-FER at 810 °C site

x/a

y/b

T1 T2 T3 T4 O1 O2 O3 O4 O5 O6 O7 O8 O1bis Co1a Co1b Co2a Co3b Na

0.1558(3) 0.0826(2) 0.2726(2) 0.3251(1) 0 0.2479(4) 0.1003(3) 0.2002(3) 0.25 0.1560(3) 0.1164(2) 0.3211(2) 0 0 0.235(2) 0.387(2) 0.049(1) 0.062(1)

0 0.2014(3) 0 0.2020(2) 0.2212(7) 0 0.0898(5) 0 0.25 0.2742(4) 0.2495(3) 0.0908(3) 0.184(6) 0 0.189(3) 0.055(3) 0.270(1) 0.418(2)

T1-O3 ) 1.638(7) [x2] T1-O4 ) 1.556(6) [x2] T2-O1 ) 1.579(3) T2-O3 ) 1.603(6) T2-O7 ) 1.618(4) [x2] T2-O1bis ) 1.574(3)

T3-O2 ) 1.631(4) T3-O4 ) 1.610(6) T3-O8 ) 1.596(4) [x2] T4-O5 ) 1.606(2) T4-O6 ) 1.592(2) T4-O7 ) 1.573(4) T4-O8 ) 1.606(3)

Co1a-O3 ) 2.28(1) [x4] Co1a -O1bis ) 2.59(1) [x2] Co1b-O5 ) 2.07(1) [x2] Co1b-O6 ) 1.91(4) Co2a-O6 ) 2.53(6) Co2a-O8 ) 2.31(3) [x2] Co3b-O6 ) 2.17(5) Co3b-O7 ) 2.05(3) Na-O7 ) 2.76(4) Na-O8 ) 2.68(5)

a

z/c

fractions

Uiso (100 Å2)a

0 0 0.2906(4) 0.2029(3) 0 0.5 0 0.1756(9) 0.25 0.5 0.1774(6) 0.2513(5) 0 0.028(2) 0.5 0 0.390(4) 0.042(8)

1.000 1.000 1.000 1.000 0.80(1) 1.000 1.000 1.000 1.000 1.000 1.000 1.000 0.21(1) 0.197(1) 0.030(1) 0.026(1) 0.024(1) 0.038(1)

6.60(4)a 6.60(4)a 6.60(4)a 6.60(4)a 8.46(8)a 8.46(8)a 8.46(8)a 8.46(8)a 8.46(8)a 8.46(8)a 8.46(8)a 8.46(8)a 8.46(8)a 10(3) 13(2) 14(2) 15(1) 11(2)

Uiso coupled during the refinement.

sites correspond, respectively, to the β-type and γ-type sites suggested by Kaucky´ et al.,27 on the basis of spectroscopic studies on dehydrated Co-exchanged ferrierites. However,

according to Kaucky´ et al., the Co2+ ions located in site β are fourfold coordinated to the O3 framework oxygens alone. The XRD results indicate that the cobalt ions in site Co1a induce a

12978 J. Phys. Chem. B, Vol. 107, No. 47, 2003

Figure 6. Plot of the variation of cobalt sites occupancies in ferrierite cage as a function of temperature.

deformation of the six-ring window, suggesting that Co2+ ions tend to reach optimal Co-O bond distance also from the two O1 oxygens of the six-ring. Figure 6 reports the distribution of Co cations in the Co1, Co1a, and Co1b positions at different temperatures. In the range 100-200 °C, some incongruencies in the sum of the occupancies of these sites can be observed. However, we are dealing with transient situations, as occurs in the heating process of Co-FER up to 810 °C, which make it difficult to accurately determine the location and occupancy of scarcely populated cation sites.

Dalconi et al. As concerns the second cobalt site (Co2) located at 53 °C near the center of the wide 10-ring channel, at increasing temperature Co ions migrate along the a axis toward the wall of the 10-ring channel, maintaining their coordination with W5 and W6. Above 350 °C, the two water molecules are lost, and the cations tend to migrate toward the four O8 framework oxygens delimiting the 10-ring channels, even if at long bond distances (Figure 7). To reach a better coordination, however, Co cations shift in a direction orthogonal to the (010) plane to a new site, called Co2a, which is threefold coordinated with one O6 and two O8 framework atoms (see Table 4). Site Co2a at 810 °C accounts for about 0.2 Co atoms per unit cell and roughly corresponds to the R-type cation site of Kaucky´ et al.27 As pointed out in the previous paragraphs, the remaining cobalt ions occupy at low temperature a third cation site (Co3) situated in the 10-ring channel, not far from the center of the 8-ring window delimiting the ferrierite cage from the 10-ring channel. At about 300 °C, some of these cations transfer to a new site, called Co3a, which is bonded with one O1 and two O7 framework oxygens, whereas the residual Co moves by about 0.5 Å to maintain its coordination with residual W3 and W5. Above 400 °C, when the dehydration is complete, the initial Co3 and the new Co3a sites are no longer occupied and a new Co3b site, coordinating framework oxygens O7 and O8 of the

Figure 7. Schematic representation of Co2 site location as obtained from Rietveld refinements of Co-FER around 500 °C (a) and Co-FER at 810 °C (b); Co3b (c) and Na (d) site location in Co-FER at high temperature.

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Figure 8. Plot of the variations of the water molecules per unit cell as a function of temperature.

eight-ring window, is populated (Figure 7). Co3b corresponds to the Cu(1) site in dehydrated Cu-ferrierite21 and is not too far from site K(1), the most populated one in the dehydrated K-exchanged ferrierite.20 Already at about 300 °C, more than 75% of the water molecules are lost, while roughly the same fraction of Co cations migrate to new sites coordinated only to framework oxygens, leaving the residual cobalt ions bonded only to the residual H2O (see Table 3). The Rietveld refinement of Co-FER at high temperature clearly shows the presence of a site located in a general position at quite long distances from framework oxygens O7 and O8 (∼2.7 Å) (site Na in Table 4). Such a site obviously cannot be attributed to either Co ions or water molecules. This site has also been found in Ni-exchanged ferrierite heated at 600 °C (and tentatively attributed to residual water). The nature of site Na is quite unclear as it is too far from Co cations to be interpreted as a water molecule and too close to water molecules to be considered a cation site. In our opinion, the most reasonable hypothesis is that this site is occupied by the residual, not exchanged, Na atoms (see Table 1). To support this hypothesis, we can observe that the Na site is very close to W4 in all hydrated forms of ferrierite. This site is present in hydrated Mg-rich natural ferrierites16 and is characterized by a very strong anisotropic displacement, with the highest rms displacement equal to 0.82 Å. According to Alberti et al.,16 “static disorder rather than effective thermal motion is probably responsible for this effect”. It is therefore likely that cations and water molecules alternate in this site; if so, water molecules are lost upon heating, whereas residual cations remain in about the same position. The strong decrease in the occupancy of site W4 during heating (see Tables 2-4) is in agreement with this hypothesis. On the whole, the occupancy refinements of the water sites (W1-W6) show that the water loss is almost completed below 400 °C; only W3 is still weakly occupied at this temperature (Figure 8). The coordination of W3 to the residual cation in sites Co1 and Co3 can explain why the water molecules in this site are lost at a higher temperature than the other ones. Figure 8 shows clearly that dehydration of Co-ferrierite is a one-step process, as also evidenced by the TG curve of Co-FER,28 which indicates a progressive and continuous loss of H2O as a function of temperature. If we now compare the Co-exchanged ferrierite at 810 °C with the Ni-exchanged one at 600 °C23 a strong analogy is evident; in both cases there are five cation sites (including the Na site) in about the same positions with similar occupancies and coordination. The only remarkable difference is that the

Ni-form does not show the presence of the O1bis framework site, so that site Na1a is only fourfold-coordinated to O3 framework oxygens. Framework Distortions. As pointed out before, most of the contraction of parameter a occurs below 400 °C. This result can be explained by the dehydration process which causes a squashing of the 10-ring channel along the a direction. In fact, the O8-O8 distance along a, which at 53 °C is 6.78 Å, decreases by about 0.11 Å at 380 °C. As there are two 10-rings in the unit cell along a, the total variation between 53 °C and 380 °C is 0.22 Å, in good agreement with the decrease of 0.20 Å registered by parameter a. This mechanism does not remarkably affect parameter b. Indeed, the decrease by about 0.05 Å of the O3-O3 distance in the b direction (because of the interaction of O3 with Co1a, see Figure 5) is compensated by a similar increase of the O8-O8 distance along b which can be attributed to the expulsion of water molecule W6; in fact, at low temperature the distance of W6 from two symmetry-related O8 of the 10-ring channel (2.63 Å) suggests a hydrogen bond O8-W6-O8. Vice versa, the b decrease is due to the shortening of the O3-O7 distancesmore accentuated with the increase of the temperatureswhich could be due to their interaction with cation site Co3b. Conclusions By continuously monitoring the thermal behavior of ferrierite, it is evident that Co-exchanged ferrierite belongs to the group of zeolites that do not undergo any significant modifications of the framework upon dehydration and that it likely maintains its capacity to re-hydrate at room conditions (see the description introduced by Alberti and Vezzalini29). Upon heating to 810 °C, Co-FER behaves as a noncollapsible structure,26 displaying only a slight contraction of the unit cell volume (-2.35%). From the structure refinements, five cation sites have been localized in dehydrated Co-FER which closely resemble those found in dehydrated Ni-exchanged ferrierite. Our results partly differ from those deduced by other techniques (spectroscopic or computational),9,27 as they suggest the presence of two other possible cation sites (Co3b and Na sites in Table 4) located in positions which may act as Lewis’ acid active species. Moreover, the R-type site is six-coordinated (not four-coordinated) to all the oxygens of the six-ring delimiting the ferrierite cage, whereas the γ-type site is out of the (100) plane in order to have a better coordination distance (for a Co cation) with the framework oxygen atoms delimiting the 10-ring channel.

12980 J. Phys. Chem. B, Vol. 107, No. 47, 2003 Acknowledgment. We acknowledge the valuable help of Dr C. Meneghini in running the GILDA beamline and of Prof. P. Ciambelli for supplying the samples and the chemical analysis. The Ministero della Universita` e della Ricerca Scientifica e Tecnologica is thanked for its financial support to the research program “Zeolites, materials of interest for industry and environment: synthesis, crystal structure, stability and applications” (COFIN 01). References and Notes (1) Li, Y.; Armor, J. N. Appl. Catal., B: EnVironmental 1993, 3, L1. (2) Bellussi, G.; Rigutto, M. S. Metal ions associated to molecular sieve frameworks as catalytic sites for selective oxidation reactions. In Studies in Surface Science and Catalysis; van Bekkum, H., Flanigen, E. M., Jacobs, P. A., Jansen, J. C., Eds.; Elsevier: Amsterdam, 2001; Vol. 137, p 911. (3) Sobalı´k, Z.; Deˇdecˇek, J.; Kaucky´, D.; Wichterlova´, B.; Drozdova´. L.; Prins, R. J. Catal. 2000, 194, 330. (4) Kaucky´, D.; Vondrova´, A.; Deˇdecˇek, J.; Wichterlova´, B. J. Catal. 2000, 194, 318. (5) Li, Y.; Armor, J. N. J. Catal. 1994, 150, 376. (6) Li, Y.; Slager, T. L.; Armor, J. N. J. Catal. 1994, 150, 388. (7) Mortier, W. J. Compilation of extraframework Sites in Zeolites; Butterworth Sci. Ltd.: London, 1982. (8) Fripiat, J. G.; Galet, P.; Delhalle, J.; Andre´, J. M.; Nagy, J. B.; Derouane, E. G. J. Phys. Chem. 1985, 89, 1932. (9) McMillan, S.; Broadbelt, L. J.; Snurr, R. Q. J. Phys. Chem. B 2002, 106, 10864. (10) Sˇponer, J. E.; Sobalı´k, Z.; Leszczynski, J.; Wichterlova´, B. J. Phys. Chem. B 2001, 105, 8285. (11) Dalconi, M. C.; Alberti, A.; Cruciani, G.; Ciambelli, P.; Fonda, E. Siting and coordination of cobalt in ferrierite: XRD and EXAFS studies at different Co loadings. Micropor. Mesopor. Mater. 2003, 62, 191. (12) Pascarelli, S.; Boscherini, F.; D’Acapito, F.; Hrdy, J.; Meneghini, C.; Mobilio, S. J. Synchr. Rad. 1996, 3, 147.

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