Reversible channel deformation of zeolite omega during template degradation highlighted by in situ time-resolved synchrotron powder diffraction

Microporous and Mesoporous Materials 104 (2007) 257–268 www.elsevier.com/locate/micromeso

Reversible channel deformation of zeolite omega during template degradation highlighted by in situ time-resolved synchrotron powder diffraction Annalisa Martucci

b

a,*

, Maria de Lourdes Guzman-Castillo b,1, Francesco Di Renzo b, Franc¸ois Fajula b, Alberto Alberti a

a Dipartimento di Scienze della Terra, Sezione di Mineralogia, Petrografia e Geofisica, Via Saragat 1, 44100 Ferrara, Italy Laboratoire de Mate´riaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRS-ENSCM-UM1, Institut Charles Gerhardt, FR 1878, ENSCM, 34296 Montpellier, France

Received 17 October 2006; received in revised form 14 February 2007; accepted 15 February 2007 Available online 25 February 2007

Abstract The thermal dehydration and the degradation of template of zeolite omega, a synthetic analog of mazzite Na6.6TMA1.8(H2O)22.2[Al8.4Si27.6O72]-MAZ, was studied in situ by synchrotron powder diffraction. The evolution of the structural features was monitored through 25 structure refinements in the temperature range from 30 to 830 C by full profile Rietveld analysis performed in the P63/mmc space group. Structural refinements allowed the steps of degradation of tetramethylammonium (TMA), the evolution of the occupation of the different water sites and the migration of the Na cations located along the axis of the 12-ring channel towards a new site near the walls of the ring to be monitored. Transient deformations of the framework were highlighted, related to the constrained diffusion of the products of dehydration and degradation through the 8-ring channels. Permanent deformations of the frameworks correspond to the removal of the TMA cations from the gmelinite cages, which induces a relaxation of the strained 180 T1–O2–T1 angles, a widening of the opening of the 8-ring channels and a star-shaped deformation of the 12-ring channels.  2007 Elsevier Inc. All rights reserved. Keywords: Zeolite omega; Thermal activation; Template degradation; In situ powder diffraction; X-ray synchrotron radiation; Rietveld analysis

1. Introduction The activation of zeolites is based on the thermal desorption or decomposition of template molecules. The microporosity of the activated zeolites is basically a replica of the volume originally occupied by the template. Nevertheless, the framework of tetrahedra which surrounds the porosity is not perfectly rigid, and can bend significantly when the template is extracted. In the case of many

*

1

Corresponding author. Tel.: +39 0532 974730; fax: +39 0532 293752. E-mail address: [email protected] (A. Martucci). Present address: Istituto Mexicano del Petroleo, Mexico D.F., Mexico.

1387-1811/$ - see front matter  2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.02.040

aluminophosphate [1] and some aluminosilicate zeolites [2], some bonds are broken and the connectivity of the framework atoms is altered by the activation process. For many aluminosilicate zeolites, albeit the topology of the bonds between lattice-forming tetrahedra is unaffected by the activation, bond angles and cell parameters are generally modified by the extraction of the template. A wellknown example is represented by the change of symmetry and pore shape of ZSM-5 when the tetrapropylammonium template is burnt or other organics are adsorbed [3–7]. In the last years, in situ time-resolved diffraction methods have significantly improved the monitoring of zeolite activation processes. Crystallographic studies have been devoted to the deformation of the framework during the

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dehydration of the cations of several natural zeolites, such as scolecite–mesolite [8], thomsonite [9], edingtonite [10], laumontite [11], brewsterite [12], phillipsite [13], stilbite [14], yugawaralite [15], epistilbite [16], bikitaite [17], and mordenite [18]. In the case of synthetic zeolites, most studies deal with the shift of the cation positions during the dehydration, for instance in the cases of the zeolites Sr-X [19], Ba-Y [20], Cs-Y [21], Ca-A [22], or Pb- and CdRHO [23]. The study of the effect of dehydration on the crystal structure of zeolite-like materials has provided a number of results of industrial interest, such as the change of pore size of the ETS-4 adsorbents as a function of temperature [24,25]. Zeolite omega is intermediate between the aluminiumrich alkali-templated zeolites and the silica-rich organictemplated zeolites, as both sodium and tetramethylammonium cations take part in the crystallization. Zeolite omega is usually crystallized in mild hydrothermal conditions (around 80–150 C) in the system (TMAOH)–NaOH– Al2O3–SiO2–H2O, where TMA is the tetramethylammonium cation [26]. Zeolite omega is a large-pore material known for its strong Brønsted acidity and its activity as an alkylation and isomerisation catalyst in petrochemical reactions [27–30]. It is recognized that zeolite omega features the same framework topology as mazzite (framework type MAZ, according to the classification by the International Zeolite Association), a natural zeolite [31–33]. Its real symmetry is P63/mmc, the same as the topological one. The alumino-silicate framework consists of gmelinite-type cages which are linked in columns parallel to the c-axis, sharing their 6-membered rings of tetrahedra. Alternate columns are staggered by one-half of the period along c-axis and are connected laterally by 5-rings to form a hexagonal assemblage of columns. Two different types of channels parallel to [0 0 1] are present: the largest channels are composed of 12-rings surrounded by gmelinite cages, while smaller channels are formed by distorted 8-rings between adjacent pairs of gmelinite cages. TMA molecules are located inside the gmelinite cage and have two possible orientations, which are symmetric to the plane passing through the center and orthogonal to the threefold axis of the cage. Sodium cations are distributed over two sites: one, at the center of the 8-ring channel, hosts most of Na and is 8-fold coordinated to six framework oxygens and two water molecules, while the other, located along the axis of the 12-ring channel, coordinates nine H2O molecules [33]. The TMA cations in zeolite omega (as in offretite, TMA-sodalite and zeolite alpha) are occluded in cages from which they cannot be extracted without decomposition. The snug fit of TMA in gmelinite cages was demonstrated by 13C NMR spectroscopy [34,35]. This confinement effect was shown to significantly modify the course of degradation of quaternary ammonium cations [36,37]. The crystal structure of mazzite after calcination at 600 C was investigated by Rinaldi et al. [38]. Analogies and differences with the results of our in situ study will be discussed in the following sections.

This investigation strives to give a more exhaustive picture of omega dehydration, with particular focus on dynamic and transient effects, using Rietveld structure analysis of temperature-resolved powder diffraction data collected using synchrotron radiation. Such experimental conditions are ideal for rapid collection of the diffraction data necessary to monitor each step of the dehydration process in detail. It is noteworthy that the response to heating is an important aspect of natural and synthetic zeolites, and knowledge of their thermal behavior affects topics ranging from their industrial applications to their identification. 2. Experimental 2.1. Materials The synthesis of the zeolite omega was carried out in a stirred autoclave at 105 C from a synthesis batch of composition 0.48 Na2O/0.024 TMA2O/0.075 Al2O3/SiO2/26.6 H2O [39]. The composition of the synthesized sample was jNa6.6TMA1.8(H2O)22.2j[Al8.4Si27.6O72]-MAZ. Na, Al, Si, C and N contents were determined by elemental analysis at the Service Central d’Analyse of the CNRS in Solaize, France, and the water content was determined by thermogravimetry. Thermogravimetric analysis of the as-synthesized sample was carried out (using a heating rate of 5 C/min) on a Setaram TG 111 thermal balance, under a constant flux of air or helium. The evolved gases were analyzed on-line using a Baltzers OME 125 mass spectrometer. 2.2. Diffraction methods A powder sample of zeolite omega-MAZ, the same used for high-resolution X-ray powder diffraction study of Ref. [33], was selected for this experiment. Time resolved diffraction data were collected on the GILDA beamline at ESRF (Grenoble). The powder sample was loaded and packed in a 0.3 mm diameter Lindemann capillary, open at both ends, and heated in situ up to 830 C using a hot air stream. The heating rate was 5 C/min. The capillary sample was mounted on a standard goniometer head and kept spinning during data collection in parallel beam Debye–Scherrer geometry, using a Si(1 1 1) monochromatized wavelength ˚ . During the heating process, powder difof 0.68765(1) A fraction patterns were recorded on the 4 mm slit-delimited portion of a translating flat image plate [40], which had a translation rate with respect to the temperature increase of 2.5 pixel/C (Fig. 1). External standard LaB6 was used to calibrate the wavelength, as well as to determine the zero-shift position, sample to detector distance, and tilting angle of the image plate detector. A whole of 48 onedimensional powder patterns were extracted from the image plate by integration of constant temperature slices (temperature variable ± 5 C and temperature step separation 20 C), using a locally adapted routine.

A. Martucci et al. / Microporous and Mesoporous Materials 104 (2007) 257–268

Fig. 1. Evolution of the XRPD patterns in the 3–18 2h interval as a function of the temperature (30–830 C) during the in situ experiment.

2.3. Structure refinement The structure refinements by full profile Rietveld analysis were performed in the P63/mmc space group by the

259

GSAS package [41], starting from site positions of framework atoms from Martucci et al. [33]. Since no evidence was found to support a change in symmetry on the powder pattern until 830 C, the same P63/mmc space group as at room temperature was adopted in all the crystal structure refinements. The Bragg peak profile was modeled using a pseudo-Voigt function with a 0.01% cut-off of the peak intensity. The instrumental background was empirically fitted using a Chebyschev polynomial of the first kind with 24 variable coefficients. The 2h-zero shift was accurately refined in all the patterns of the data set. One scale factor and the unit-cell parameters were allowed to vary for all histograms. In the final cycles, the refined structural parameters for each data histogram were the following: fractional coordinates for all atoms, isotropic displacement and occupancy factors for extraframework sites and isotropic displacement factors (one for all tetrahedral cations, for all framework oxygen sites, for all Na cations, for all water molecules and another for TMA molecules). Soft constraints were imposed on T–O distances during the initial cycles, and then released in the final cycles of refinement. Extraframework sites were deduced from difference Fourier synthesis, and labeled according to the notation of Martucci et al. [33]. Fig. 2 shows the final observed and

Fig. 2. Observed (crossed) and calculated (solid line) diffraction patterns and difference curves from Rietveld refinements of zeolite omega at 30 C (a), 355 C (b), 515 C (c), and 725 C (d).

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Table 1 Lattice parameters and refinement details for omega zeolite at 30, 355, 515 and 725 C, respectively

Space group ˚) a (A ˚) c (A ˚ )3 V (A Refined pattern 2h range () Rwp (%) Rp (%) R2F (%) Nobs Nvar

30

355

515

725

P63/mmc 18.230(1) 7.640(1) 2198.8(3) 2.2–18

P63/mmc 18.142(1) 7.658(1) 2182.7(3) 2.9–18

P63/mmc 18.112(1) 7.642(1) 2171.1(3) 2.9–18

P63/mmc 18.087(3) 7.619(1) 2158.8(6) 3.2–18

4.00 3.00 4.65 1497 83

4.4 3.2 7.4 1497 83

5.1 3.8 8.2 1487 66

4.5 3.4 9.6 1477 56

˚. Notes:PSynchrotronP radiation, k = 0.68765(1)A Rp ¼ ½Y io  Y ic = Y io . P P Rwp ¼ ½ wi ðY io  Y ic Þ2 = wi Y 2io 0:5 . P P 2 2 2 2 RF ¼ j F o  F c j = j F o j. Estimated standard deviations in parentheses refer to the last digit.

calculated powder patterns for zeolite omega at 30, 355, 515 and 725 C. The refinement parameters are reported in Table 1, the refined coordinates in Tables 2a and 2b, bond distances and angles in Table 3. Table 2b Atomic coordinates, thermal parameters and site occupancies of extraframework atoms for zeolite omega at 30, 355, 515 and 725 C, respectively

355

515

725

T1

x/a y/b z/c Uiso

0.1570(4) 0.4873(6) 1/4 0.025(1)

0.1558(3) 0.4887(8) 1/4 0.034(4)

0.1553(3) 0.4923(1) 1/4 0.032(4)

0.1553(4) 0.4877(4) 1/4 0.049(4)

T2

x/a y/b z/c Uiso

0.3549(3) 0.0941(4) 0.0447(8) 0.025(1)

0.3526(3) 0.0871(5) 0.0405(8) 0.034(4)

0.3593(4) 0.1014(4) 0.0361(6) 0.032(4)

0.3559(4) 0.0976(4) 0.0439(6) 0.046(4)

O1

x/a y/b z/c Uiso

0.2591(7) 0.5181(14) 1/4 0.038(2)

0.2563(11) 0.5126(23) 1/4 0.042(5)

0.2516(18) 0.5031(24) 1/4 0.042(5)

0.2564(14) 0.5128(24) 1/4 0.098(5)

O2

x/a y/b z/c Uiso

0.4216(4) 0.8432(8) 1/4 0.038(2)

0.4119(6) 0.8237(13) 1/4 0.042(5)

0.4056(8) 0.8113(16) 1/4 0.042(5)

0.4109(8) 0.8219(14) 1/4 0.098(5)

x/a y/b z/c Uiso

0.3868(10) 0.1001(10) 1/4 0.038(2)

0.3770(15) 0.0986(15) 1/4 0.042(5)

0.3692(22) 0.0974(19) 1/4 0.042(5)

0.3849(14) 0.1000(16) 1/4 0.098(5)

O4

x/a y/b z/c Uiso

0.4365(5) 0.1069(5) 0.0716(9) 0.038(2)

0.4414(7) 0.1113(8) 0.0641(12) 0.042(5)

0.4440(10) 0.1104(9) 0.0645(16) 0.042(5)

0.4387(6) 0.1073(8) -0.0668(14) 0.098(5)

O5

x/a y/b z/c Uiso

0.1588(2) 0.3176(4) 0.0091(22) 0.038(2)

0.1686(3) 0.3371(6) 0.0073(31) 0.042(5)

0.1550(3) 0.3098(6) 0.0078(40) 0.042(5)

0.1504(4) 0.3008(6) 0.0055(34) 0.098(5)

O6

x/a y/b z/c Uiso

0.2712(7) 0 0 0.038(2)

0.2612(65) 0 0 0.042(5)

0.2900(17) 0 0 0.042(5)

0.2783(14) 0 0 0.098(5)

O3

Note: Estimated standard deviations in parentheses refer to the last digit. Site fractions are fixed to 1.0 for all framework atoms.

355

515

725

1/2 0 0 0.21(2) 0.67(3)

1/2 0 0 0.17(8) 0.61(3)

0.088(2) 0.212(3) 1/4 0.14(2) 0.02(1)

0.101(4) 0.194(4) 1/4 0.21(2) 0.12(2)

0.1089(4) 0.2177(4) 1/4 0.17(8) 0.32(6)

0.524(5) 0.049(11) 1/4 0.16(4) 0.25(4)

Na1

x/a y/b z/c Uiso Frac

1/2 0 0 0.13(1) 0.82(1)

1/2 0 0 0.14(2) 0.64(3)

Na2

x/a y/b z/c Uiso Frac

0 0 0.205(28) 0.13(1) 0.20(2)

0 0 0.141(2) 0.14(2) 0.14(2)

Na2 0

x/a y/b z/c Uiso Frac

W1

x/a y/b z/c Uiso Frac

0.533(1) 0.067(1) 1/4 0.08(1) 0.82(1)

0.551(2) 0.101(4) 1/4 0.14(4) 0.37(3)

W4

x/a y/b z/c Uiso Frac

0.547(3) 0.393(2) 1/4 0.08(1) 0.176(12)

0.622(1) 0.414(1) 1/4 0.14(4) 0.08(7)

W5

x/a y/b z/c Uiso Frac

0.012(1) 0.123(2) 0.097(1) 0.08(1) 0.56(1)

0.066(3) 0.135(2) 0.147(2) 0.14(4) 0.05(4)

W6

x/a y/b z/c Uiso Frac

0.087(1) 0.173(1) 1/4 0.08(1) 0.57(2)

0.105(3) 0.209(3) 1/4 0.14(4) 0.38(6)

N

x/a y/b z/c Uiso Frac

1/3 2/3 3/4 0.09(2) 0.97(5)

1/3 2/3 3/4 0.14(5) 0.53(2)

C1

x/a y/b z/c Uiso Frac

0.289(1) 0.578(1) 0.157(3) 0.09(2) 0.72(2)

0.294(1) 0.588(2) 0.162(3) 0.14(5) 0.62(2)

0.302(4) 0.605(6) 0.182(6) 0.17(5) 0.25(3)

C2

x/a y/b z/c Uiso Frac

1/3 2/3 0.013(16) 0.09(2) 0.70(2)

1/3 2/3 0.058(13) 0.14(5) 0.59(2)

1/3 2/3 0.313(17) 0.17(5) 0.18(3)

Table 2a Atomic coordinates and thermal parameters of framework atoms for zeolite omega at 30, 355, 515 and 725 C, respectively 30

30

Note: Estimated standard deviations in parentheses refer to the last digit.

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Atoms

30 C Distance

355 C

515 C

725 C

T1–O1 T1–O2 T1–O4 [x2] T2–O3 T2–O4 T2–O5 T2–O6 T–O–T T1–O1–T1 T1–O2–T1 T2–O3–T2 T1–O4–T2 T2–O5–T2 T2–O6–T2

1.653(2) 1.662(2) 1.644(3) 1.657(3) 1.647(4) 1.653(8) 1.664(3) Angle 145.7(21) 179.8(11) 142.3(13) 138.1(10) 133.7(10) 132.7(10)

1.650(2) 1.648(3) 1.650(3) 1.650(2) 1.650(3) 1.656(3) 1.650(3)

1.650(1) 1.633(2) 1.650(1) 1.650(1) 1.650(1) 1.650(2) 1.650(1)

1.650(3) 1.665(3) 1.649(2) 1.649(2) 1.649(2) 1.732(3) 1.649(2)

153.8(32) 157.5(18) 153.1(19) 138.5(12) 155.5(14) 116.6(10)

168.1(41) 142.8(19) 164.4(27) 133.3(12) 118.3(11) 156.4(24)

152.4(32) 155.2(18) 144.4(22) 135.2(10) 114.1(12) 143.2(22)

Gmelinite cage O1–O2 O3–O3 O4–O4

5.13(1) 5.95(2) 6.50(1)

4.89(2) 6.25(2) 6.35(2)

4.83(4) 6.50(3) 6.28(3)

4.84(4) 5.97(3) 6.37(3)

12-ring channel O5–O5 O6–O6

10.03(1) 9.89(2)

10.59(2) 9.48(2)

9.72(3) 10.20(3)

9.42(2) 10.07(2)

8-ring channel O2–O2 O4–O4 O6–O6

6.25(2) 5.55(1) 8.34(1)

6.73(3) 5.51(2) 8.66(2)

7.05(5) 5.40(3) 7.61(3)

6.75(5) 5.44(4) 8.02(4)

Note: Estimated standard deviations in parentheses refer to the last digit.

3. Results and discussion 3.1. Thermal gravimetry and analysis of the evolved gases

0

-5

-5

-10

-15

mass %

0

counts (a. u.)

mass %

The thermogravimetric curve in flowing air for zeolite omega is illustrated in Fig. 3. The weight loss of nearly

12% observed at temperature lower than 300 C is usually attributed to the loss of the water molecules coordinating the sodium cations. At higher temperature, much slower weight loss is observed, until very rapid weight loss with the maximum rate at 535 C completes the degradation of the organics. The total weight loss is 17.8% of the initial mass. The curve of evolution of water at mass 18 is also reported in Fig. 3. The detection of water is slowly delayed by comparison with the thermogravimetric curve due to reversible retention in a filter between the sample and the mass spectrometer. If this effect is taken into account, the evolution of water vapor qualitatively parallels the loss of mass. The curve of evolution of CO2 at mass 44 is also reported in Fig. 3. This indicates that some organics are oxidized between 350 and 420 C. A fast evolution of CO2 around 530 C corresponds to the final step of weight loss. The results of thermal gravimetry and CO2 evolution are in excellent agreement with the differential thermal analysis data of Araya et al., who observed the endothermic effects corresponding to loss of water at temperature lower than 300 C and two exothermic effects at higher temperature, a shallow effect from 300 to 475 C and a sharp effect from 475 to 575 C [42]. The thermogravimetric curve in flowing helium of zeolite omega is reported in Fig. 4. The earlier stages of activation in an inert atmosphere are very similar to activation in air. The main degradation step takes place at a slightly higher temperature (nearly 550 C) and corresponds to a lower weight loss in helium than in air. The loss of mass is completed by a slower phenomenon between 570 and 750 C. The curve of evolution of N2 at mass 28 and NH3 at mass 17 are shown in Fig. 4. A fast evolution of nitrogen and ammonia was observed between 535 and 575 C. Ammonia was the main nitrogen-containing molecule observed by thermal pyrolysis of zeolite omega under vacuum by Cole and Kouwenhoven [43]. The evolution of

counts (a. u.)

Table 3 ˚ ) and angles () within the framework of zeolite Selected bond distances (A omega at 30, 355, 515 and 725 C, respectively

261

-10

-15

-20

-20 0

100

200

300

400

500

600

700

800

0

100

200

T/ºC TG

mass 18

300

400

500

600

700

800

T/ºC mass 44

Fig. 3. TG curve (full line) and corresponding mass spectroscopy data at masses 18 (H2O, void symbols) and 44 (CO2, full symbols) of zeolite omega (heating rate 10 C/min) under flux of air.

TG

mass 17

mass 28

Fig. 4. TG curve (full line) and corresponding mass spectroscopy data at masses 17 (NH3, void symbols) and 28 (N2, full symbols) of zeolite omega (heating rate of 10 C/min) under flux of helium.

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ammonia in the temperature range 575–610 C only accounts for a small fraction of the mass loss, which continues up to 750 C. This suggests that the final loss of mass in inert atmosphere essentially corresponds to the degradation of carbonaceous residues. It is interesting to note that the TG analyses performed in air or helium medium are very similar up to 550 C but differ markedly in the range 550–650 C. This result is obviously related to the different gas flow, and the acceleration of the degradation of the template in air flow clearly depends on the availability of a more favorable decomposition pathway through the formation of oxidized products. The formation of CO2 from the methyl groups of the template, as observed in air flow, is easier than the formation of ethylene in an inert atmosphere. The carbonaceous residues formed by the polymerization of a fraction of ethylene are only degraded at higher temperature. However, it is worth pointing out that the decomposition of the organics at 800 C is complete both in the air and helium flow, as indicated by the total mass loss in excellent agreement with the value calculated by the occupancies of TMA and water sites of the structure refinement at 30 C (17.8%). 3.2. Refinement by in situ X-ray data: temperature-dependent variation of unit cell parameters Unit cell parameters at some representative temperature levels are reported in Table 1. The parameters of the P63/ ˚ and mmc cell at room temperature are a = 18.230 A ˚ , and the cell volume is 2198.8 A ˚ 3. The variab = 7.640 A tions in unit cell parameters in the temperature range 30– 830 C are indicated in Fig. 5 as fractions of the values at room temperature. Below 200 C, small variations in cell parameters are observed. Up to about 100 C, parameter a slightly increases whereas c remains substantially unchanged. Above 100 C, the parameter a remains stable, while c slightly decreases. The cumulate effect of these variations

parameter/parameter(0)

1.005

account for a 0.1% decrease in cell volume from 30 to 200 C. It appears that the loss of about half the water content of the zeolite causes no significant cell parameter variation. Further dehydration, in the 200–375 C temperature range, corresponds to a remarkable change in unit ˚ ), cell parameters. The a value decreases of 0.5% (0.08 A whereas the variation of c is characterized by a sudden change of slope and its value increases almost to 0.4% ˚ ). The increase in c fails to fully compensate the (0.03 A decrease in a and the unit cell volume decreases by about 0.6%. When the temperature rises from 375 to 480 C, parameter c remains more or less constant, while parameter a and the unit cell volume slowly decrease. The coefficients of linear expansion are 8.8 · 106 in direction a and 2.1 · 106 in direction c. A sharp decrease in cell size takes place in the temperature range 480–550 C, in correspondence with the degradation of TMA. Parameter c decreases by about 0.25% and parameter a by about 0.1%, with a corresponding decrease in cell volume of about 0.35%. The cell size decreases with a more moderate slope in the 550–830 C temperature range, where no further loss of mass is observed. The crystal structure does not show any proof of collapse at least up to 830 C. The coefficients of linear expansion of the template-free zeolite omega in this temperature range are 6.5 · 106 in direction a and 1.26 · 105 in direction c. If these values are compared with the values measured in the 375–480 C range, when TMA was still present, it can be observed that the presence of the template does not affect the thermal shrinking in directions a and c in the same way. The negative thermal expansion coefficient in direction a is scantily affected by the presence of the template, while the presence of TMA strongly reduces cell shrinkage in direction c. Indeed, in the presence of TMA, cell shrinkage in direction c is one fourth of the shrinkage in direction a, while, in the absence of TMA, shrinkage in direction c is twice the shrinkage in direction a. The evolution of the cell parameters ties in with previous data measured at room temperature on samples activated at several temperature levels, once allowance is made for thermal expansion [44]. 3.3. Refinement by in situ X-ray data: temperature-dependent occupation of the extraframework sites

1 0.995 0.99 0.985 0.98 0

100

200

300

400

500

600

700

800

T/ºC a

c

V

Fig. 5. Evolution of unit cell parameters a (void squares) and c (void triangles) and cell volume (full circles) with temperature. Initial values ˚ , c = 7.64 A ˚ , V = 2198.8 A ˚ 3. a = 18.23 A

The structure refinements during the temperature ramp show large modifications in the occupancy of the extraframework sites, due to the processes of dehydration and degradation of TMA. The temperature dependence of the occupation of the water sites is illustrated in Fig. 6. Dehydration begins to be observed beyond 90 C, which is consistent with thermogravimetric data. The dehydration patterns largely differ from one water site to another. Occupation of the site W5, in the main 12-ring channel in front of the 4-ring ladder of the gmelinite cage, decreases proportionally compared to the rise in temperature until complete

A. Martucci et al. / Microporous and Mesoporous Materials 104 (2007) 257–268

and Vezzalini [45] by crystal energy calculations, the other, with higher occupancy, is bonded to the cations located at the center of the 8-ring channel. This site is the same as the W1 site in zeolite omega in this temperature range. The position of Na2 cations is affected by the loss of their coordination water. The fractions of sodium cations detected in the Na1 and Na2 sites are indicated in Fig. 7 as a function of temperature. At room temperature, the Na cations in the main channel are in the Na2 site, along the axis of the 12-ring, and are only coordinated by the water molecules W5 and W6. As mentioned above, dehydration of the 12-ring channel (the W5 and W6 sites) is completed at around 375 C. In correspondence with this final step of dehydration of the main channel, the Na cations migrate from the Na2 site towards a new site, Na2 0 , located near the walls of the channel. This site is coordinated with two framework oxygens O5 at a quite short dis˚ ) and with four framework oxygens O6 at tance (2.40–2.50A ˚ ). This position is near to a very large distance (3.20–3.30 A the K site found by Rinaldi et al. in mazzite heated at 600 C [38]. The migration of the Na cations from Na2 to Na2 0 site is only completed at about 500 C. This suggests that some disordered water is still present in the 12-ring channels in the 375–500 C temperature range. The location of the site Na1, in the 8-ring channels, is not affected by dehydration. However, the intensity of the detected signal somewhat decreases at approximately 240 C, in correspondence with the beginning of the loss of water from the site W1. The 8-ring channels of zeolite omega are occupied by a continuous chain of alternating sodium cations Na1 and water molecules W1 [33]. Partial dehydration of the W1 sites could induce some disorder in the positions of the sodium cations and decrease the measured occupancy of the Na1 site. The occupancies of the sites of the template molecule are indicated in Fig. 8 as a function of temperature. Structure refinements show that the state of the template

fraction / fraction (30 ºC)

1 0.8 0.6 0.4 0.2 0 0

100

200

300

400

500

600

temperature (°C) w1

w5

w4

263

w6

Fig. 6. Evolution of the occupation of the water sites as a function of temperature. Sites W1 (filled squares), W4 (filled triangles), W5 (empty triangles), and W6 (empty squares).

dehydration at 375 C. The W4 site, in the 8-ring window of the gmelinite cage, and the W6 site, in the 12-ring channel in front of the 5-ring ladder, follow a more complex dehydration pattern: a partial loss of water from 90 to about 240 C is followed by a plateau until the remaining water is suddenly lost between 355 and 375 C. Water is more strongly retained in the W1 site, sandwiched between two Na cations in the 8-ring channel: the occupation of this site remains constant up to about 240 C. At higher temperature, the occupation of the site decreases by about two-thirds up to 375 C. The residual water is retained until dehydration is suddenly completed around 530 C. The dehydration of zeolite omega seems to require less severe conditions than the dehydration of mazzite, its natural analog. In mazzite heated at 600 C [38] about 80% of water molecules were lost. The residual H2O molecules are in two partially occupied sites: one, weakly occupied, is coordinated to the Mg site, as demonstrated by Alberti

Atoms per unit cell (a.u.c.)

6 5 4 3 2 1 0 30

130

230

330

430

530

630

730

Temperature (°) Na1

Na2

Na2'

Fig. 7. Evolution of the occupation of the sodium sites as a function of temperature. Sites Na1 (filled squares), Na2 (filled triangles), and Na2 0 (empty triangles).

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quent decrease of the measured occupancies. It is interesting to observe that the occupancy of the C1 site, pointing towards the 6-ring of the gmelinite cage, is much less altered than the occupancy of the N site, at the centre of the cage, and the C2 site, pointing towards the 4-rings of the cage. This provides clues about the orientation of the trimethylamine molecule in the gmelinite cage. In effect, the degradation of the fraction of TMA alters the periodicity of the template sites. However, the methyl groups of trimethylamine still contributes to the periodicity of the TMA molecules. All the N and C sites are emptied in the temperature range 480–550 C, in correspondence with the completion of the thermal degradation of TMA.

fraction / fraction (30 ºC)

1 0.8 0.6 0.4 0.2 0 0

100

200

300

400

500

600

temperature (ºC) N

C1

C2

Fig. 8. Evolution of the occupation of the template sites as a function of temperature. Sites N (filled squares), C1 (empty circles), and C2 (empty triangles).

3.4. Refinement by in situ X-ray data: temperature-dependent deformations of the framework

is significantly altered in the temperature range 280–375 C. At 375 C, the occupancy of the N site is reduced by about 50% and a remarkable decrease of the occupancy of the C sites is observed. This phenomenon can be related to the first step of the Hoffmann degradation, in which TMA is transformed in trimethylamine. The loss of a methyl group produces ethylene in a reducing and anhydrous environment or methanol and CO2 in an oxidizing and hydrous environment [43]. In both cases, residual protons compensate the charge unbalance due to the Hoffmann degradation of TMA to trimethylamine. The evolution of CO2 beyond 350 C when the zeolite is heated in an air flow (Fig. 3) is consistent with the oxidative mechanism of degradation, albeit the amount of CO2 measured by mass spectroscopy suggests that only a small fraction of TMA reacted at temperature below 500 C. The transformation of a fraction of TMA into protonated trimethylamine severely disrupts the periodicity of the residual template molecules, with a conse-

Structural refinements during the thermal ramp provide information about how the occupancy of the extraframework sites affects the geometry of the framework. The evolution of the shape of the gmelinite cages during the thermal ramp can be defined by characteristic distances, illustrated in the graph of Fig. 9. The size of the 8-ring window of the cage is defined by the distances between oxygens O2–O2, O3–O3, and O4–O4 (see Fig. 9). The O1–O2 distance defines the size of the 6-ring window between gmelinite cages. The O2–O2 distance corresponds to parameter c of the unit cell (its evolution with temperature is illustrated above in Fig. 5). The modifications of parameter c do not ˚ . Much more exceed 0.3% of the initial value of 7.62 A important deformations of the cage take places in the orthogonal direction. In fact, the O3–O3 distance undergoes transient expansions which attain 10% of the initial ˚ . Expansion events take place at around value of 5.95 A 330 and 530 C and the O3–O3 distance exactly recovers

O1

7.9

O2

O4

7.4

T2 O3

O3

O3

OO4 4 O 2 O2

O-O distance (A)

O4

T1

6.9 6.4 5.9 5.4 4.9 4.4 30

130

230

330

430

530

630

730

Temperature (ºC)

O4-O4

O3-O3

O1-O2

Fig. 9. Variations of the O–O distances in the gmelinite cage and schematic representation of the cage.

O2-O2

A. Martucci et al. / Microporous and Mesoporous Materials 104 (2007) 257–268

its initial value after each event. The value at 815 C is ˚ , significantly the same as the initial one. The expan5.94 A sion bursts of the 8-ring window corresponds to the main events of degradation of the organics in the cage (Fig. 8). It is tempting to attribute the transient expansion of the 8-ring window to the pressure of the molecules produced by the degradation of the template. The gmelinite cages are not in communication with the main 12-ring channels and the volatile products formed inside have to diffuse along the 8-ring channels, occupied by the chains of Na1 cations and W1 water molecules. The slow diffusion of the decomposition products develops a significant internal pressure and causes the corresponding transient expansion of the 8-ring windows. The shift in position of the O3 atoms corresponds to a rotation of the T2 tetrahedra and a corresponding shift of the O4 atoms (see Fig. 9). As a consequence, the O4–O4 distance undergoes two transient shrinkages in correspondence with the expansion events of the O3–O3 distance. After the deformation events the O4– O4 distance does not completely recovers its initial value ˚ ) and retains some permanent shrinkage (5.44 A ˚, (5.55 A see Table 3). The O1–O2 distance also retains a permanent deformation, after its decrease (between 230 and 350 C) ˚. from an initial value of about 5.15 to about 4.90 A Fig. 10 illustrates the evolution of the 8-ring channels parallel to [0 0 1], through which the degradation products have to diffuse. The O2–O2 and O6–O6 distances, which delimit the 8-ring, begin to increase at 240 C. The evolution of the lesser dimension of the channel, the O2–O2

265

distance, is especially significant, as this distance limits the diffusion of the degradation products of the template and water molecules W1 and W4. The O2–O2 distance ˚ at 240 C to a first peak of 6.98 A ˚ increases from 6.05 A ˚ at 390 C and a second peak of 7.05 A at 515 C. After this 16% increase, the O2–O2 distance settles at a final value of ˚ . In correspondence with the deformation of about 6.65 A the ring, the T1–O2–T1 angle (which at 30 C is near to 180) decreases to 150, and the T2–O6–T2 angle decreases from 135 to 115. The latter angle is very narrow, even if not the narrowest found in dehydrated zeolites. In fact a T–O–T angle of 114 has been found in dehydrated natrolite (metanatrolite) [46]. A Si–O–Si angle of 116 has also been found in bavenite a framework-like berillo-aluminosilicate [47], whereas angles near 120 exist in other dehydrated zeolites, e.g. 122 in Ba-phillipsite [13], or 118 and 122 in epistilbite [16]. It must be pointed out that in omega zeolite the narrow angle occurs during a transient deformation of the framework, whereas in the other cases cited above the narrow angles are present in permanent deformations of the framework. At temperature higher than 350 C, the T2–O6–T2 angle relaxes and assumes a final value of 145. The relaxation of the angle corresponds to a shift of the O6 oxygens and an evolution the O6–O6 ˚ at approxidistance which, after a maximum of 8.68 A mately 350 C, decreases towards a final value of about ˚ . A particularly interesting parameter is the ratio 8A between the O6–O6 and the O2–O2 distances, which correspond to the two axes of the elliptical 8-ring (Fig. 10). At 9

T1

O2

O4

O-O distance

8

T1

6 5

O4

T2

7

T2 4 30

O6

130

230

330

O2-O2

T2

430

530

630

730

Temperature (°C)

O6

O6-O6

O4-O4

T2 O4

O4

190

O2

T1

T-O-T angle (º)

180

T1

170 160 150 140 130 120 110 100 30

130

230

330

430

530

630

730

Temperature (ºC) T1-O2-T1

T1-O4-T2

T2-O6-T2

Fig. 10. Variations of the T–O–T angles and O–O distances in the 8-ring channel parallel to [0 0 1] and schematic representation of the 8-ring.

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room temperature this ratio has a value of 1.31, corresponding to quite an elongated ellipse. At 515 C, in correspondence with the final degradation of the template, a ˚ ) is transient maximum of the O2–O2 distance (7.05 A accompanied by a transient minimum of the perpendicular ˚ , see Table 3). In this configuration, O6–O6 distance (7.61 A the ratio between the axes of the ellipse reaches a value of 1.08 and the 8-ring channel becomes almost circular, a shape which allows the degradation products to transit more easily towards the exterior of the crystal. It should be noted, however, that after the water molecules W1 and W4 and degradation products of TMA went through the 8-ring channel the O2–O2 distance remains by far larger than in the as-synthesized phase (see Table 3). As a con˚ at 30 C sequence, the Na1–O2 distance increases by 3.12 A ˚ to 3.38 A at 725 C, which is too large to be assumed as a coordination distance. Consequently, the Na1 site, which at RT is eightfold coordinated, if we consider the large Na1–O2 as a bond distance (Na1–O4 = 2.78 [x4], Na1– ˚ [x2]), becomes fourfold W1 = 2.18 [x2], Na1–O2 = 3.12 A ˚ coordinated (Na1–O4 = 2.72 A [x4]) in an unusual square coordination. This finding could be explained through the presence of Brønsted acid sites. In the previous section it was observed that the charge imbalance due to the Hoffmann degradation of TMA, which is about 20% of the charge imbalance due to the tetrahedral Al, could be compensated by residual protons. It is easy to infer that the increase of the Na1–O2 distance at high temperature is due to the formation of an O2–H Brønsted sites. This explains not only the geometry of the 8-ring, in particular the shortening of the O6–O6 distance, but also the modifications undergone by the 12-ring channel, which will be discussed later.

The structural modifications of the 8-ring channel are paralleled by the deformation of the 12-ring channel represented in Fig. 11. In fact, cell parameter a corresponds to the sum of the O6–O6 distances through the 8-ring ˚ (Fig. 10) and the 12-ring (Fig. 11) channels. The 0.38 A increase in the O6–O6 distance through the 8-ring (Fig. 10) in the 200–355 C temperature range is more than ˚ decrease in the O6–O6 distance compensated by a 0.45 A through the 12-ring (Fig. 11), with a net decrease of ˚ of the a parameter of the cell (Fig. 5). The shape 0.07 A of the 12-ring is significantly modified in this temperature range: the decrease of the O6–O6 distance from 9.93 to ˚ is accompanied by an increase of the O5–O5 dis9.48 A ˚ to 10.7 A ˚ , inducing an important startance from 10.1 A shaped deformation of the 12-ring (Fig. 11). Consequently, the T2–O5–T2 and T2–O6–T2 angles dramatically modify their values, which at 200 C are quite similar (135) whereas at 355 C become 155 and 115, respectively. The 200–355 C temperature range corresponds to the progressive loss of water from the sites W5 and W6, in the 12-ring channel, and the star-shaped deformation can be attributed to an adaptation of the framework to the decreasing occupation of the channel volume by the water molecules. At 375 C, the Na atoms of the Na2 site begin to migrate to the new site Na2 0 (Fig. 7). Interaction with the Na2 0 cations draws the O5 oxygens towards the centre of the channel, dramatically modifying the geometry of the 12-ring, which suddenly loses its star-shaped deformation and resumes a circular section (Fig. 11). Between 460 and 515 C, in correspondence with the final degradation of the organic template, the shape of the main channel changes again: the 12-ring undergoes a reverse star-shaped deformation, with the O6 oxygens pushed farther from the

O-O distance

11 10.5 10 9.5 9 30

130

230

330

430

530

630

730

Temperature (ºC) O6-O6

O5-O5

T-O-T Angle (˚)

160 140 120 100 80 30

130

230

330

430

530

630

730

Temperature (ºC) T2-O5-T2

T2-O6-T2

Fig. 11. Variations of the T–O–T angles and O–O distances in the 12-ring channel parallel to [0 0 1] and schematic representation of the 12-ring.

A. Martucci et al. / Microporous and Mesoporous Materials 104 (2007) 257–268

centre of the channel. This behaviour differs from that shown by mazzite dehydrated at 600 C. In this phase the 12-ring channel has a star-shaped form with O5–O5 and ˚ , respectively. The extraO6–O6 distances 10.23 and 9.94 A framework K site localized in the large channel, which can be favourably compared with the Na2 0 site in dehydrated omega, is bonded, with almost ideal distances, both to ˚ [x2]) and O6 (2.60 A ˚ [x2]) [38]. Also, the 12-ring O5 (2.78 A channel of dehydrated omega is deformed in the shape of a star while the O5–O5 distance is by far shorter than the O6–O6 distance and, consequently Na2 0 site is at a short ˚ [x2]) but a large distance from distance from O5 (2.43 A ˚ O6 (3.26 A [x4]). We can easily infer that the different behaviour is related to the distortion of the 8-ring due to the O2-H Brønsted acid site. 3.5. Refinement by in situ X-ray data: transient vs. permanent deformations of the framework Thanks to in situ collection of diffraction data, it is possible to monitor transformation processes in real time. This is an obvious advantage when fast-occurring phenomena have to be monitored, but can be a doubtful advantage when the rate of temperature rise and the rate of transformation are comparable. Such was the case as far as the diffusion of the degradation products of zeolite omega in our experiment is concerned. The rise in temperature was fast enough to keep the system far from the equilibrium state during the main steps of dehydration and degradation of the organic template. In our experiment, this effect was a significant benefit because it highlighted non-equilibrium phenomena, such as the swelling of the 8-rings of the gmelinite cage and the channels parallel to [0 0 1] under the pressure of the outgoing molecules. It can be observed that, once the main degradation steps are completed, the geometry of the framework significantly differs from the initial one. These differences are related to the presence of the tetramethylammonium and water molecules in the as-synthesized zeolite. The removal of these molecules prevents a complete return to the original geometry of the framework. The subsequent relaxation of the framework is related to specific features of the structure of mazzite and zeolite omega. It is especially relevant that the framework oxygen O1, which lies on the 6-ring of the gmelinite cage (Fig. 9), interacts, both in hydrated and dehydrated mazzite as well as in as-synthesized omega, with the extraframework ions located inside the gmelinite cage [33]. As a consequence, the 6-ring is characterized by a strong ditrigonal distortion; with T1–O1–T1 angle around 150 and T1–O2–T1 very near to 180 (Table 3). As far as in 1961, Liebau [48] posed the question whether or not straight Si–O–Si bridging bonds can be present in silicates. The conclusion was that Si–O–Si angles of 180 are energetically unfavorable. Alberti [49] showed that every time the topological symmetry of a zeolite imposes framework oxygens on centers of symmetry, and consequently T–O–T angles of 180 are present, the true symme-

267

try is lower than the topological one and no framework oxygen lies on a center of symmetry, so that no straight T–O–T bonds are present, because such angles, and by extension, very large angles are energetically unfavorable. In activated zeolite omega, in which TMA is no longer present, no extraframework cation interacts with the framework oxygens of the 6-ring of the gmelinite cage (O1 and O2). Therefore, both T1–O1–T1 and T1–O2–T1 angles assume values near 155 (Table 3), which are energetically more favorable. As a consequence of the shift of O2 towards the center of the 6-ring of gmelinite cage, the O2–O2 distance in the 8-ring channel parallel to [0 0 1] increases and the O6–O6 distance decreases, so that the 8-ring maintains a shape more circular than in the as-synthesized material (Fig. 10). Our hypothesis is that the shift of O2 is guided by an O2-H Brønsted acid sites, as a result of the presence of protons, residual to the Hoffmann degradation of TMA. As said before, the O6–O6 distance in the 8-ring is inversely related to the same distance in the 12-ring, thus explaining why in activated mazzite the O6–O6 distance is larger than the O5–O5 distance, in contrast with the results found in both hydrated and dehydrated form of mazzite and as-synthesized omega. 4. Conclusions The high quality of the XRPD data collected during the in situ time resolved heating process allowed careful investigation of the structural changes occurring during template decomposition and water removal in zeolite omega. Albeit the topology and the symmetry of the structure were not altered by the activation process, permanent and transient deformations of the framework were highlighted. The former are due to the removal of the interactions between guest molecules and zeolite. The latter are due to the pressure of volatile molecules striving to leave the crystal through small openings and can only be observed in nonequilibrium conditions. In effect, the decomposition products of the TMA in the gmelinite cage and the water molecules from the sites W1 and W4 have no access to the main 12-ring channels and can only leave the crystal through the 8-ring channels parallel to [0 0 1], in which their diffusion is severely hindered by the Na1 cations. As a consequence, the 8-rings underwent a transient swelling in correspondence with the fastest losses of mass during the activation process, viz. at 300–350 and 500–550 C. It is interesting to observe that the degradation of TMA begins well before the end of the dehydration and that partial Hoffmann degradation of the TMA to trimethylamine accounts for a disordering of the template at a temperature as low as 280 C. The permanent deformations related to the activation process affect the porosity of zeolite omega and present a practical interest. The main channels of assynthesized zeolite omega are delimited by a nearly circular 12-ring. The opening of the ring can be lessened by starshaped deformations, both at intermediate levels of

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dehydration and, more significantly, once the TMA has been removed. As a consequence, the pore opening of the main channel of zeolite omega heated at 550 C is about ˚ , instead of the 7.2 A ˚ observed in dehydrated mazzite. 6.7 A This effect confirms the influence of the extraframework cations on the porosity of zeolites, because the shape of the main channels is affected by the occupation of cation sites both inside and outside the 12-ring. The shape of the main channel is clearly affected by the position of the Na cation inside the 12-ring, which move from the Na2 site at the center of the ring to a new Na2 0 site near the wall of the channel when water is extracted. However, the degradation of TMA in the gmelinite cage also influences the star-shaped deformation of the main channel, triggering a rotation of tetrahedra which affects the geometry of the 8-ring and 12-ring channels. As it is sometimes forgotten, the structure of a zeolite can only be apprehended as a global system, in which the position of each atom exerts a far-reaching influence on the geometry of the whole framework. Acknowledgments The authors are grateful to Philippe Gonzalez for the thermogravimetric experiments. We are also indebted to Carlo Meneghini (University of Rome) and Giuseppe Cruciani (University of Ferrara) for their assistance during the experiments at the BM08 (GILDA) beamline (ESRF, Grenoble, France) and the processing of the Translating Imaging Plate data. The Italian CNR and INFM are also acknowledged for providing financial support to GILDA and its associated facilities. References [1] A. Martucci, A. Alberti, Stud. Surface Sci. Catal. 155 (2005) 19. [2] E. Bourgeat-Lamy, P. Massiani, F. Di Renzo, P. Espiau, F. Fajula, T. Des Courie`res, Appl. Catal. 72 (1991) 139. [3] H. van Koningsveld, H. van Bekkum, J.C. Jansen, Acta Crystallogr. B 43 (1987) 127. [4] H. van Koningsveld, J.C. Jansen, H. van Bekkum, Zeolites 10 (1990) 235. [5] B.F. Mentzen, Mater. Res. Bull. 27 (1992) 831. [6] B.F. Mentzen, Mater. Res. Bull. 30 (1995) 1333. [7] B.F. Mentzen, F. Lefe`bvre, J. Chem. Phys. 95 (1998) 1052. [8] K. Sta˚hl, R. Thomasson, Zeolites 14 (1994) 12. [9] K. Sta˚hl, Mater. Sci. Forum 378 (2001) 346. [10] K. Sta˚hl, J.C. Hanson, Eur. J. Mineral. 10 (1998) 221. [11] K. Sta˚hl, G. Artioli, J.C. Hanson, Phys. Chem. Minerals 23 (1996) 328. [12] K. Sta˚hl, J.C. Hanson, Microp. Mesop. Mater. 32 (1999) 147. [13] A. Sani, G. Cruciani, A.F. Gualtieri, Phys. Chem. Minerals 29 (2002) 351. [14] G. Cruciani, G. Artioli, A. Gualtieri, K. Sta˚hl, J.C. Hanson, Amer. Miner. 82 (1997) 729.

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