Dealumination of HZSM-5 via steam-treatment

Microporous and Mesoporous Materials 164 (2012) 9–20

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Dealumination of HZSM-5 via steam-treatment Lay Hwa Ong, Márta Dömök, Roberta Olindo, André C. van Veen, Johannes A. Lercher ⇑ Technische Universität München, Lehrstuhl II für Technische Chemie, Lichtenbergstr. 4, D-85747 Garching, Germany

a r t i c l e

i n f o

Article history: Available online 26 July 2012 This paper is dedicated to Jens Weitkamp at the occasion of his 70th birthday. Keywords: Zeolite steaming Tetrahedral extra framework aluminum Site 27 Al MAS NMR

a b s t r a c t Two distinct locations for tetrahedral aluminum in HZSM-5 have been identified, showing bond angles of 150° and 155° T–O–T, respectively. The former site is more abundant and has been indirectly associated with aluminum in isolated positions. Upon steaming its concentration decreases, following a first order rate law and leading to the formation of tetrahedrally coordinated extra-lattice aluminum as well as to invisible extra-lattice aluminum. The latter is speculated to be also tetrahedrally coordinated and kinetically linked to the visible portion of the extra-lattice aluminum. Both types of extra-lattice aluminum neutralize lattice charge and lead to a decrease of the concentration of bridging Si–OH–Al groups, which is initially more pronounced than the loss of lattice aluminum. With steaming duration the concentration of Brønsted acid sites stabilizes indicating that the extra-lattice aluminum atoms begin to form larger clusters at a rate equivalent to the rate of dealumination. The lattice aluminum with the more obtuse T–O–T angle is stable under the steaming conditions chosen. As its concentration is nearly equivalent to the concentration of aluminum sites sufficiently close to exchange Co2+ ions, it is inferred that aluminum resisting dealumination constitutes these sites. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction Zeolites are tectosilicates, in which frequently a fraction of the Si–O tetrahedra is replaced by tetrahedra containing for example a three valent cation such as Al3+. The substitution has to be compensated by a cationic species, ranging from alkali cations and protons to multivalent metal cations. The catalytic activity of a zeolite towards converting organic molecules such as hydrocarbons shows in general a direct relation to the concentration of Brønsted acid sites, which are generated by framework aluminum atoms [1]. Hence, dealumination – the process removing framework aluminum atoms from the zeolite – adversely affects the rate of reactions catalyzed by protons. This process might induce a change in the properties of the remaining lattice aluminum atoms and acid sites and has to be understood in quantitative and qualitative terms in order to explain and predict changes in the catalytic properties. Water is able to hydrolytically remove aluminum atoms from their tetrahedral coordination in the lattice of the zeolite. The enormous importance of this process and the implications upon the stability and catalytic properties of the resulting zeolites caused an intense exploration of the associated phenomena over time and we only cite a few key investigations of the dealumination kinetics [2–5]. Fast and slow dealumination steps have been iden⇑ Corresponding author. Address: Department Chemie, TU München, Lichtenbergstr. 4, 85748 Garching, Germany. Tel.: +49 89 28913540; fax: +49 89 28913544. E-mail address: [email protected] (J.A. Lercher). 1387-1811/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2012.07.033

tified and several dealumination models have been proposed for different zeolite topologies. Despite these efforts, the reasons for the preference of particular sites to be dealuminated and the nature of the species generated in the process have not been comprehensively described and discussed. This has led to a situation in which the individual steps of the overall process are still not unequivocally explained. Some authors conclude the extraction of aluminum from the framework to occur via a penta-coordinated aluminum species identified by a signal at 30 ppm in the 27Al MAS NMR spectrum [6]. Others suggest the intermediate species being distorted tetrahedral aluminum species [7], which could, however, also be the precursors to the penta-coordinated aluminum species. It is interesting to note that also the nature of the resulting extra-framework aluminum species is not unequivocally accepted. Up to three different kinds of octahedral aluminum species have been described using data from 27Al MAS NMR spectroscopy [8]. In addition, several models for reversible Al framework coordination have been discussed for zeolites such as HZSM-5, HMOR, HBEA and HY [9– 11]. Comparing the vast number of papers proposing dealumination models, it is important to keep in mind that the dealumination process is influenced by the zeolite structure, the aluminum distribution and its ability to stabilize the resulting defect sites. Often, proposed models are based on zeolites with high aluminum content, which contain primarily aluminum tetrahedra that are less stable against hydrolysis and dealumination [12]. Therefore, it is to be explored whether or not these models are quantitatively

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applicable to highly siliceous zeolites, the structure of which is more stable against steam. Considering the complexity of the issues outlined above, we opted to explore the dealumination of HZSM-5 and to investigate the gradual change of the framework Al and the integrity of the zeolite structure with steaming duration. The choice of a highly siliceous material is motivated by the possibility to study the dealumination of acid sites, which are often assumed to be mostly isolated and non-interacting. We will use in-depth physicochemical analysis in parallel to the kinetic evaluation in order to understand the individual processes. 2. Experimental methods 2.1. Sample preparation and steaming procedure NaZSM-5 was synthesized by mixing colloidal silica, Al(NO3)3 9H2O, NaOH and TPABr in order to obtain a reaction mixture with a composition of 100 SiO2: 0.1–0.2 Al2O3: 5 Na2O: 10 TPABr: 4000 H2O. After aging, the resulting gel was kept at 453 K for 48 h in an autoclave; then filtered and washed until pH 8 was reached. Drying of the product at 100 °C for 10 h was followed by thermal treatment, first in He with an increment of 1 K/min to 673 K holding at that temperature for 3 h, then rising the temperature in air to 793 K with the identical rate and holding the temperature for 3 h to remove the organic template. The hydrogen form was obtained by triple ion-exchange with NH4NO3 solution and subsequent calcination in CO2 free air (increment of 1 K/min to 793 K and holding the temperature for 3 h. According to atomic absorption spectroscopic (AAS) analysis, samples prepared by this procedure had a Si/Al ratio of 87, corresponding to an aluminum concentration of 174 lmol/g. For the steaming procedure, 500 mg zeolite (355–500 lm particle size) was heated in a vertical tubular quartz reactor to 723 K under nitrogen flow. N2 was then switched to 0.333 mol h1 water (pure steam) delivered by a Gilson 307 HPLC pump. A layer of SiC on top of the catalyst assured a constant vaporization of the injected water. The duration of steaming was varied from 2 to 48 h. 2.2. Characterization 2.2.1. IR spectroscopy The concentration of acid sites was determined by IR spectroscopy using pyridine as probe molecule. Spectra were recorded with a Perkin Elmer 2000 spectrometer at a resolution of 4 cm1. The samples were pressed into self-supporting wafers and activated at 723 K for 1 h in vacuum. After cooling to 423 K the spectrum of the activated sample was recorded first. Then pyridine was adsorbed in small dosages until full saturation of the bridging OH group at 3606 cm1 was observed. The system was equilibrated for 0.5 h then evacuated and the spectrum was recorded. The concentration of Brønsted and Lewis acid sites (referred to as BAS and LAS) was estimated from the areas of the bands at 1515–1565 and 1435–1470 cm1, respectively. 2.2.2. MAS NMR spectroscopy MAS NMR spectra were acquired on a Bruker Avance AMX-500 spectrometer at room temperature with a magnetic field of 11.75 T. Samples were packed into ZrO2 rotors and spun at 15 kHz. For 27Al MAS and MQMAS NMR measurements the samples were hydrated for at least 48 h. Al(NO3)39H2O was used as reference. An excitation pulse with power level of 7 dB and a length of 0.6 ls was applied for the 1D spectrum. The relaxation time was 250 ms. For 1D spectra, 2400 scans were recorded. MQMAS spectra were recorded with a sequence of three pulses

[13]. The power level was 7 dB for the first two pulses and 35 dB for the last one. The pulse lengths were p1 = 8 ls, p2 = 3.2 ls and p3 = 52 ls. The evolution time t1 was incremented in intervals of 1 ls and data were processed with Bruker Topspin. After Fourier transformation, the 2D spectra were sheared so that the orthogonal projection on the isotropic axis gave the 1D spectrum free of any anisotropic broadening [14]. For quantification of the 27Al MAS NMR spectra, the chemical shift and the quadrupolar coupling constant (QCC) were obtained from the MQMAS spectrum and used to deconvolute the 1D spectra. It has been demonstrated that all types of aluminum occurring in zeolites with quadrupole coupling constants are detected with NMR field strengths above 17.25 T corresponding to proton resonance frequencies above 750 MHz or echo-experiments [15,16]. The restrictions to the 500 MHz instrument in this work prevented resolving all (broad) signal contributions. This fraction of typically highly distorted aluminum oxygen moieties has been labeled ‘‘invisible’’ extra-framework aluminum and has been assessed via the aluminum mass balance. For 1H MAS NMR spectra, samples were activated in vacuum at 673 K for 14 h to eliminate adsorbed water. At the magnetic field of 11.75 T the Larmor frequency for 1H was 500 MHz. Adamantane (C10H16) was used as reference material for the calibration of the chemical shift. For recording spectra, an excitation pulse with a power level of 6.00 dB and a length of 1.60 ls was applied. The relaxation time was 2 ms and 100 scans were recorded for all spectra. For 29Si MAS NMR the Larmor frequency was 99.36 MHz. The reference for the measurements was solid Si(OSi(CH3)3)4. An excitation pulse with a power level 7 dB and a length of 0.6 ls was applied. The relaxation time was 250 ms. For each spectrum 2400 scans were recorded. 2.2.3. X-ray diffraction (XRD) X-ray powder diffraction patterns were recorded using a Philips X0 Pert Pro System operating with a CuKa1-radiation (0.154056 nm) at 40 kV/40 mA. Measurements were performed on a spinner with a 1=4 00 slit from 5° to 50° 2h (0.05° min1). The relative crystallinity of steamed samples was determined by measuring the intensity of the diffraction signal of the (051) peak and comparing it to that of the reference unsteamed sample. 2.2.4. Nitrogen physisorption Surface area, pore volume and pore size distribution of the materials were obtained by nitrogen adsorption on a PMI automated BET sorptometer. Brunauer, Emmet and Teller (BET), Barrett, Joyner and Halenda (BJH; desorption branch), and t-plot methods were used for calculating specific surface area and meso- and micro-pore volume. Before N2 adsorption the sample was activated in vacuum at 673 K for 2 h. After activation, the weight of the dried sample was determined. Subsequently, the sample was cooled to 77 K and liquid nitrogen was adsorbed at increasing partial pressures. 2.2.5. Na+ and Co2+ ion exchange The concentration of isolated and close Al sites was determined by subsequent ion exchange with Na+ and Co2+ adapted from Ref. [17]. Briefly, 100 ml of 0.5 M NaCl per g H-ZSM-5 served in a 3 times repeated ion exchange for 8 h at room temperature. Following triple washing a second ion exchange step was repeated 3 times using 150 ml of 0.05 M Co(NO3)26H2O per g Na-ZSM-5 at room temperature for 12 h. Following triple washing the sample was dried at 383 K overnight. The adopted simplified approach used atomic absorption spectroscopic (AAS) analysis to quantify the concentrations of Na+ and Co2+ present in the zeolite. The concentration of isolated Al BAS sites was taken equal to the

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Scheme 1 and discussed below. Differential equations corresponding to the respective mechanisms (Table 1) were solved by numerical integration accounting for the associated kinetic parameters. The estimation of kinetic parameters involved an iterative process minimizing an objective function quantifying the mismatch between experimental results and model predictions throughout the investigated steaming time. The objective function was defined as non-weighed sum of squares of the deviations between experimentally determined and model predicted concentrations of the considered Al species. The parameter estimation imposed as additional constraints that all rate constants have positive values for the sake of physically meaningfulness.

Scheme 1. Scheme of the simplified mechanisms for the initial dealumination process assuming (a) a sequential transformation of the Al species at 57 ppm via an NMR invisible Al species into the Al species detected at 60 ppm and (b) a parallel transformation of the Al species at 57 ppm into an NMR invisible Al species and the Al species detected at 60 ppm.

3. Results concentration of Na+ and the concentration of close Al BAS sites was calculated as twice the Co2+ concentration after Na+ and subsequent Co2+ exchange.

3.1. IR spectroscopy The effect of steaming duration on the IR spectra of the samples activated at 723 K is presented in Fig. 1a. The band at 3606 cm1, attributed to Brønsted acid sites, strongly decreased within the first 19 h steaming and then further weakened slowly. The band at 3740 cm1, which is attributed to terminal SiOH groups, did not change significantly with steaming time. The broad band

2.3. Kinetic evaluation of dealumination kinetics during steaming Kinetic modeling of the dealumination process was performed assuming to two potential reaction mechanisms summarized in

Table 1 Rate equations and mass balances for the considered initial dealumination mechanisms and estimates for the rate constants obtained upon parameter optimization. Reaction step

Rate constant/h1

Rate equation

(a) Sequential transformation of the Al species detected at 57 ppm (peak IV1) Al (57 ppm) ? Al (invis) r1 = k1 [Al (57 ppm)] Al (invis) ? Al (57 ppm) r1 = k1 [Al (invis)] Al (invis) ? Al (60 ppm) r2 = k2 [Al (invis)] Al (60 ppm) ? Al (invis) r2 = k2 [Al (60 ppm)] Mass balance Al (57 ppm) Al (invis) Al (60 ppm)

8.54  102 1.67  101 9.27  102 9.36  102

R (57 ppm) = r1 + r1 R (invis) = r1  r1  r2 + r2 R (60 ppm) = r2  r2

(b) Parallel transformation of the Al species detected at 57ppm (peak IV1) Al (57 ppm) ? Al (invis) r1 = k1 [Al (57 ppm)] Al (invis) ? Al (57 ppm) r1 = k1 [Al (invis)] Al (57 ppm) ? Al (60 ppm) r2 = k2 [Al (57 ppm)] Al (60 ppm) ? Al (57 ppm) r2 = k2 [Al (60 ppm)] R (57 ppm) = r1 + r1  r2 + r2 R (invis) = r1  r1 R (60 ppm) = r2  r2

b

a

3606

0.01

3738

Mass balance Al (57 ppm) Al (invis) Al (60 ppm)

7.21  102 1.64  101 1.38  102 0.00

0.003

5h

0.002

1454 1447

2h

0.004

0.002

1545

0h

0.006

Absorbance

Absorbance

0.008

0.001

19 24

0 3800

3700

3600

3500

3400

3300

Wavenumber (cm-1)

3200

3100

0.000 1600

1550

1500

1450

1400

-1

Wavenumber (cm )

Fig. 1. (a) IR spectra of activated steamed HZSM-5 samples. (b) Difference IR spectrum of pyridine adsorbed at 423 K on 5 h steamed HZSM-5.

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L.H. Ong et al. / Microporous and Mesoporous Materials 164 (2012) 9–20

180 Total acid sites BAS LAS

Acid sites concentration (µmol/g)

160 140 120

0h

100

2h 5h

80

19 h 60

24 h 70

40

50

30

10

-10

Chemical shift (ppm) 20 Fig. 3. 1D

27

Al MAS NMR spectrum of parent and steamed HZSM-5 samples.

0 0

8

16

24

32

40

48

Steaming time (h)

Brønsted and Lewis acid sites. The intensity of the band of pyridinium ions strongly decreased with steaming duration until around 24 h. Then, the rate of decrease was considerably lower. The concentration of Lewis acid sites remained almost constant during the steaming treatment. A very minor initial increase in the beginning is attributed to the formation of extra framework aluminum species as will be discussed later when focusing on the mechanism of dealumination. Thus, the overall decrease in the total acidity is essentially due to the decrease in the concentration of Brønsted acid sites.

Fig. 2. Changes in total, Brønsted (BAS) and Lewis (LAS) acid site concentrations as a function of steaming time calculated from IR spectra of adsorbed pyridine.

between 3630 and 3720 cm1 is attributed mainly to OH groups on extra framework aluminum and/or on aluminum partially hydrolyzed from the framework [18]. The broad band with maximum at 3250 cm1 is attributed to intramolecular hydrogen bonds between bridged hydroxyl groups and neighboring oxygen containing species. The fact that it appeared only with some samples, could also point to the presence of residual water. After adsorption of pyridine at 423 K the band of the bridging OH group disappeared completely, while the band between 3630 and 3720 cm1 remained unperturbed, indicating that the corresponding hydroxyl groups, i.e., those associated with extra lattice alumina, lack acidic character. The concentration of the coordinatively adsorbed pyridine (1442–1455 cm1) remained approximately constant, but a second signal appeared as shoulder at 1454 cm1 in the IR spectra of steamed samples besides the peak at 1447 cm1 observed with the unsteamed sample (Fig. 1b). The more complex band is attributed to pyridine adsorbed on less accessible aluminum cations and on hydroxyl groups (band at 1447 cm1 already present in the parent material) and to pyridine adsorbed on well-accessible Lewis-acidic aluminum cations at 1454 cm1 (most likely in the form of extra framework aluminum species) [19]. The concentration of pyridinium ions (1545 cm1) formed upon the adsorption of pyridine on Brønsted acid sites decreased with increasing steaming duration (Fig. 2). Fig. 2 summarizes the effect of steaming time on the concentration of

a

3.2.

Al MAS NMR spectroscopy

According to the 1D MAS NMR spectra most of the Al in the parent and steamed material adopt tetrahedral coordination as indicated by the peak between 40 and 80 ppm chemical shift (Fig. 3). Upon steaming the decrease in intensity of the peak corresponding to tetrahedral Al was not compensated by the increase in intensity for the peak related to Al in octahedral environment. Hence, the mass balance suggests the existence of Al species invisible to NMR. These ‘‘NMR invisible’’ Al species are considered to exist in a highly distorted tetrahedrally coordination, resulting in high quadrupolar coupling constants, which cause peak broadening beyond detection in the 1D spectrum. 2D MQMAS NMR was used to characterize the type of aluminum coordination in the steamed HZSM-5 samples. For all steamed samples, only tetrahedrally-coordinated aluminum atoms were observed in the MQMAS spectra, showing isotropic distributions as a result of variations in Al–O–Si bond angle and/or Al–O bond length [20]. The presence of aluminum in distorted tetrahedral

steaming time (h) 0 2 5 19 24

Intensity

27

b

F1

Peak IV3 Peak IV 1 Peak IV 2

70

60

50

40

30

F2

Chemical shift (ppm) Fig. 4. (a)

27

Al MQMAS F1 projection of HZSM-5 as function of steaming time. (b) aluminum in different T sites in the 2D MQMAS of 24 h steamed sample.

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L.H. Ong et al. / Microporous and Mesoporous Materials 164 (2012) 9–20

a

b Peak IV1

Peak IV2

Peak IV3

64

Fig. 5. Deconvolution of high resolution times with respect to (a)).

Concentration ( µmol g-1)

a 100

51

38

95

26

63

0

32

27

Al MAS NMR spectra obtained by 1D projection of 2D MQMAS spectra of (a) 5 h and (b) 24 h steamed HZSM-5 ((b) magnified 1.2

Peak IV1

Peak IV2

Peak IV3 Invisible

80 60 40 20

b 100 Concentration ( µmol g-1)

77

Peak IV1

Peak IV2

Peak IV3 Invisible

80 60 40 20 0

0 0

5

10

15

20

25

0

5

Steaming time (h)

10

15

20

25

Steaming time (h)

Fig. 6. Changes in concentration of Al species as function of steaming time calculated from 27Al MAS NMR and changes in concentration of the NMR invisible species calculated from mass balance (symbols). Lines correspond to the concentrations predicted by the kinetic model assuming a sequential conversion (a) and predictions assuming a parallel conversion (b) of Al species during steaming.

environment caused a non-symmetric interaction with the external magnetic field, resulting in a larger quadrupolar interaction. As a consequence, broadening of the contour below the isotropic line was observed. Samoson et al. [7] reported similar features in spectra of dealuminated faujasites and ZSM-5 and suggested that the signal is related to non-framework tetrahedral aluminum species. The isotropic projection in the F1 dimension gives a 1D highresolution spectrum of aluminum without quadrupolar second-order broadening. A close analysis of the asymmetric shape of the peak shows the existence of at least two types of aluminum species with tetrahedral coordination (Fig. 4a), pointing to two different types of aluminum sites in the steamed HZSM-5 [21]. The latter species experiences relatively small quadrupolar interactions and its present in comparable concentration, since the contour of the Al in the 2D MQMAS spectra lies along the isotropic line (Fig. 4b). Thus, we can assume that they have very similar MQ MAS efficiency. From the coordinates of the center of gravity of the different resonances in the 2D MQMAS spectrum, the quadrupolar coupling constants (CQCC) as well as the isotropic chemical shifts diso were calculated according to the following equations in which mL is the Larmor frequency, g is the asymmetry factor and PQ is the ‘‘Second Order Quadrupole Effect’’ (SOQE)/quadrupolar interaction product:

@ iso ¼

17@ F1 þ 10@ F2 27

ð1Þ

rffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pg ¼ C gcc

1þ

g2 3

 ¼

1=2 17 v 2L ð@ F1  @ F2 Þ 162; 000

ð2Þ

The excitation and conversion of MQMAS measurements depend on the ratio of mQ/mrf preventing that the estimates are quantitative [22]. Thus, quantification was done by fitting the corresponding F1 projection using the quadrupolar parameters determined in the MQ MAS experiment. The spectrum was deconvoluted into three peaks: peak IV1 with chemical shift of 57.2 ± 0.5 ppm, peak IV2 with chemical shift of 54.4 ± 0.5 ppm and peak IV3 with a chemical shift of 60.5 ± 0.5 ppm (Fig. 5a and b). As shown in Fig. 6 the Al represented by peak IV1 was preferentially removed, while the peak IV3 at around 60 ppm increased within 24 h steaming time. Therefore, peak IV3 was attributed to extra framework tetrahedral aluminum or partially hydrolyzed distorted tetrahedral Al. Comparing the decrease in tetrahedrally coordinated aluminum and the concentration of Brønsted sites (Fig. 7a) one notes a much more pronounced decrease of the concentration of Brønsted acid sites. As this effect was constant for all samples investigated, we conclude that nearly for each aluminum removed from the framework a further Brønsted acid site is lost through presumably coordinating a cationic aluminum oxide cluster (see Fig. 7b). By extrapolating this leads to a concentration of about 50 lmol/g of tetrahedral aluminum protected by extra-lattice aluminum oxide clusters.

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L.H. Ong et al. / Microporous and Mesoporous Materials 164 (2012) 9–20 Al

Concentration ( µmol/g)

140 120 100 80 60 40

BAS from IR

20

Tetrahedral Al from NMR (peak IV1+peak IV2)

0 0

8

16

24

Steaming time (h) Fig. 7. Comparison of BAS concentration from IR spectroscopy of adsorbed pyridine and framework Al content (peak IV1 + peak IV2) from 27Al MAS NMR.

d = 0.50a + 132 for 27Al and Sid = 0.5793a 25.44 for 29Si we use these to relate the isotropic peaks in the 27Al to those in the 29 Si MAS NMR spectra. In the 29Si MAS NMR spectrum of unsteamed HZSM-5 (Fig. 8) the peak at 106.1 ppm is related to Si with one Al in its second coordination sphere. The peaks detected at 112.3 and 115.2 ppm correspond to two groups of T sites at lower and at higher average bond angle, respectively. Using the relations according to Ref. [22], angles of 150° and 155°, respectively, were estimated. The concentrations of these two groups, i.e., 69% for the T–O–T sites giving a signal at 112.3 ppm and 26% for the sites at 115.2 ppm, agree well with the distribution of the two equivalent T sites derived from the 27Al MAS NMR (peak IV1 and IV2 in Fig. 6). Deconvolution of the 29Si MAS NMR spectra of the steamed HZSM-5 was not attempted due to the overlapping of the peak of Si(1Al) at around 106 ppm with that of Si(nOH) corresponding to defect sites at around 110 ppm. 3.4. 1H MAS NMR measurements

112.3

115.2

106.1

[ppm]

-90

Fig. 8. Deconvolution of the sample.

3.3.

-102

-114

-125

-137

29

Si MAS NMR spectrum for the unsteamed HZSM-5

29

Si MAS NMR spectroscopy

As empirical relationships between the isotropic chemical shift d and the average T–O–T angle a have been published [23,24], i.e.,

0h

2h

6.4

4.8

3.2

1.6

0.0

6.4

4.8

3.2

1.6

0.0

4.8

3.2

1.6

0.0

24h

5h

6.4

The change in the proton concentration of various hydroxyl groups in steamed samples was monitored via 1H MAS NMR spectroscopy. As illustrated in Fig. 9, the deconvolution of the spectra showed contributions of peaks at 2.0 ± 0.1, 2.5 ± 0.4, 3.0 ± 0.1 and 4.4 ± 0.2 ppm. The first peak (2.0 ± 0.1 ppm) is assigned to terminal SiOH groups on the external crystal surface. The peak at 2.5 ± 0.4 ppm is attributed to internal Si–OH groups in silanol nests, which are associated with defect sites present in the original material or formed upon dealumination [25,26]. The peak at 3.0 ± 0.1 ppm, which appeared already after short steaming periods, is attributed to framework-connected Al–OH species, which are generated by partial hydrolysis of framework Al–O bonds and which are intermediates in the dealumination process [11]. Extended steaming led to the full removal of such transient Al species, converting them into extra-framework Al species. These species may also contribute to the peak at 3.0 ± 0.1 ppm. The peak at 4.4 ± 0.2 ppm was assigned to bridging hydroxyl groups. The intensity of the bridging OH groups decreased with increasing steaming time and stabilized after 19 h steaming. Good agreement was obtained between the concentration of Brønsted acid sites quantified from IR spectroscopy of adsorbed pyridine and by evaluating the bridging OH group determined from 1H MAS NMR spectroscopy (Fig. 10).

4.8

3.2

1.6 1

0.0

6.4

Fig. 9. Deconvoluted H MAS NMR spectra of HZSM-5 after 0, 2, 5 and 24 h steaming.

Decrease of BAS in IR (%)

L.H. Ong et al. / Microporous and Mesoporous Materials 164 (2012) 9–20

15

3.7. Kinetic evaluation of the Al species conversion

100 80 60 40 20 0 0

20

40

60

80

100

1

Decrease of BAS in H NMR (%) Fig. 10. Comparison of the BAS concentrations determined by IR spectroscopy and 1 H MAS NMR.

Table 2 Textural properties of steamed HZSM-5 samples. Steaming time (h)

BET surface area (m2/g)

Micropore volume (cm3/g)

Mesopore volume (cm3/g)

0 2 19 48

335 372 341 349

0.15 0.15 0.15 0.16

0.15 0.20 0.16 0.18

The change in the intensity of the SiOH groups in terminal and defect sites observed in 1H MAS NMR spectroscopy was larger than the corresponding change in intensity from that observed in IR spectroscopic measurements. While it could be related to difficulties in defining the baseline of the SiOH groups in the IR spectra shown in Fig. 1, we would also point to the fact that the intensity of the bands in the IR spectra could be obscured by changing molar extinction coefficients. Thus, in cases of such a discrepancy, the concentrations derived from the NMR measurements are used for description of the changes. Condensation of hydroxyl groups and healing of defect sites with steaming time would eventually contribute to the overall decrease in the intensity of the SiOH groups. 3.5. N2 adsorption and X-ray diffraction Changes in textural properties induced by steaming (Table 2) were explored by N2 adsorption measurements. No direct correlation between the variation of the micropore volume and steaming conditions was observed. It remained essentially unchanged after steaming for 48 h indicating that extra framework aluminum did not block micropores. It increased slightly. This is tentatively related to restructuring of amorphous parts of the material. The diffractograms of the steamed HZSM-5 samples were almost indistinguishable from that of the parent (not shown); only a slight loss in intensity was observed for some reflexes. Selecting the unsteamed sample as reference, the loss in crystallinity after 48 h steaming was calculated to be only 6%, revealing that the structural integrity of the zeolite was largely maintained. 3.6. Ion exchange of Na+ and Co2+ The ion exchange of the of the fresh zeolite led to the incorporation of 102 lmol/g Na+ and 9 lmol/g Co2+ relating to the presence of 84 lmol/g total Al sites and 18 lmol/g close Al sites. The low concentration of Co2+ to be exchanged indicates that the majority of Al sites is isolated and that only a minor fraction is present in close distance to the next Al3+ cation.

As described above three tetrahedral aluminum species have been identified by 27Al MAS NMR spectroscopy, with peaks at approximately 54, 57 and 60 ppm. The Al species at 57 ppm (peak IV2) was hardly changed during steaming during the first 24 h. The peak at 54 ppm decreased with increasing steaming time with an exponential function, pointing to a first order decay. At the same time the concentration of the peak at 60 ppm increased almost linearly with the steaming duration. Closing the mass balance suggests the presence of invisible aluminum (hydr)oxide cations. The comparison between the decrease of the concentration of Brønsted acidic hydroxy groups and the reduction of the concentration of aluminum leads to the conclusion that invisible aluminum species must contribute to the disappearance of the bridging OH groups together with the species at 60 ppm and it is concluded that invisible aluminum is a precursor to the species causing the peak at 60 ppm. The changes are quantitatively described considering reversible and irreversible conversion routes of the Al species (Scheme 1). The differential equations corresponding to the assumed mechanisms (Table 1) involve first order kinetics in the Al species concentrations with higher orders appearing to be unsuitable in preliminary attempts. Both kinetic models attain a satisfying description of the experimental data (Fig. 6) with rate constants being generally in the same order of magnitude (Table 1). However, it is remarkable that a back-transformation of the Al species recorded at 60 ppm (peak IV3) is completely suppressed during the parameter optimization for mechanism assuming a parallel transformation of the Al species detected at 57 ppm (peak IV1). Obviously, the resulting sole consideration of a forward transformation causes the concentration of the Al species at 60 ppm to increase steadily with time.

4. Discussion 4.1. Coordination and acidity of aluminum 1

H MAS NMR and IR spectra of adsorbed pyridine indicate that 53 lmol/g of the initial 123 lmol/g bridging OH groups was retained after 24 h steaming, while 80 lmol/g of tetrahedrally coordinated framework aluminum was detected by 27Al MAS NMR (peaks IV1 and IV2) for this sample. This indicates that not all framework aluminum leads to Brønsted acidity in the dealuminated samples. Two hypotheses can be formulated to account for the difference between the NMR and IR results. The first would be that part of the Al in the framework was only partially hydrolyzed with the Al species not generating Brønsted acid sites, but maintaining their tetrahedral coordination upon hydration by water. The second (and more likely) hypothesis is that the negative charge carried by some of the aluminum atoms in the framework is neutralized by extraframework Al species. Hence, aluminum in these sites would remain tetrahedrally-coordinated, but the negative charge of the framework would not be balanced by a proton, but by an aluminum (hydr)oxide cluster. To explore the validity of the first hypothesis, we reviewed the possibility of formation of partially hydrolyzed aluminum in the form of framework Lewis acid sites. These sites are claimed to be formed during the calcination of zeolites BEA and ZSM-5 and subsequent hydration of these sites to lead to partially hydrolyzed framework Al [27]. The adsorption of one water molecule on such a framework Lewis acid site may result in species a in Scheme 2, which could account for the higher concentration of tetrahedrally-coordinated Al in the 27Al MAS NMR spectra with respect to the concentration of Brønsted acid sites determined by IR spec-

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H

a

TO

O

OT

+H2O TO

OH

Al

[Si]

[Si]

OT

TO

TO

H2O

OT

TO

Al TO

OT

OT

OT pyd

-H2O

pyd

TO

b

OH [Si] TO

Al TO

OT

OT OT

Scheme 2. Formation of tricoordinated Lewis acid sites.

troscopy. However, the adsorption of pyridine on tri-coordinated Al should also lead to an increase in the concentration of Lewis acid sites (species b in Scheme 2) in parallel to the decrease in Brønsted acid sites. As this was not observed, we conclude that the difference in the concentration of tetrahedrally coordinated aluminum and the concentration of Brønsted acid sites cannot be attributed to dehydroxylated aluminum species in the lattice. In addition, partially hydrolyzed aluminum, being connected to the framework by three or less bonds, is highly unstable. Thus, such species would be rapidly removed and partially hydrolyzed in the 24 h steamed sample, hence, it should account for only a small portion of the tetrahedral aluminum not possessing Brønsted acidity. In this context, it should also be noted that Jacobs et al. were also unable to experimentally detect tri-coordinated Al in dehydroxylated H–Y zeolite using X-ray fluorescence techniques [28]. In addition, Al K-edge XAS spectra measured at high temperatures clearly indicate that aluminum is mainly in the tetra coordinated state even at temperatures up to 975 K. The exposure to air or water directly annihilates the tri-coordinated aluminum species, which are observed only in vacuum at temperatures above 675 K [29]. Overall, the combination of MAS NMR and IR spectroscopy allows concluding that tri-coordinated Al and partially hydrolyzed Al, if formed, can exist only as transient species. Such species are hardly present after 24 h of steaming. Thus, the tetrahedrally coordinated Al detected in 27Al MAS NMR at 60 ppm (peak IV3) is attributed to stable extra-framework aluminum in tetrahedral coordination neutralizing aluminum oxygen tetrahedra in the lattice (Scheme 3). It is remarkable that the

a OH

Al

TO

Si

OT

O

OT Al

[Si] TO

b

OH2

TO

O Al

OT

OT Scheme 3. Neutralized Al sites.

O OH

Al

Al

O

O

Si

largest fraction of hydrolyzed aluminum forms such monomeric positively charged oxygen containing moieties. The wavenumber and the shape of the SiOHAl group did not change markedly upon dealumination. The comparison between the concentrations of Brønsted acid sites determined by IR and by 1H MAS NMR spectroscopy indicates, however, that the SiOHAl groups remaining after extended steaming have a slightly higher molar extinction coefficient than those of the parent material. Increasing molar extinction coefficients of OH groups are usually associated with a higher polarizability and, hence, with higher acid strength. It is unclear at present, whether these subtle variations are related to a weakening of the OH bond strength induced by the presence of extra lattice aluminum or by the selective removal of acid sites of lower strength. Let us now address the location and distribution of the labile aluminum species that was removed in the first 24 h steaming and the influence of these extra-framework species on the remaining framework Al in steamed HZSM-5. 4.2. Initial dealumination – location and distribution of more labile Al species 27 Al MAS NMR recorded throughout the initial steaming indicated the preferential removal of Al from the HZSM-5 framework with a T–O–T angle around 150° (peak 57 ppm). After the onset of the reaction, the removal of the remaining Al slowed down considerably (Fig. 6). Similar results have been reported for steamed zeolite BEA [30]. As deduced above from the stoichiometry of the removal of BAS compared to the corresponding framework tetrahedral Al, nearly all aluminum removed from the lattice acts as charge compensating cation. While it is tempting to relate this to the presence of next nearest neighbor lattice aluminum, as it has been inferred from similar observations for H-BEA [31], we would like to emphasize that the present sample does not show evidence for such sites. In contrast, the sites sufficiently close to ion exchange divalent cations such as Co2+ did not vary during steaming indicating that such sites have an unusual and unexpected stability. Before further discussing the attribution of the individual 27Al MAS NMR peaks, it should be noted that combined experimental and theoretical chemistry indicated the lack of correlation between the chemical shift and the Al siting [34]. Thus, 27Al MAS NMR spectra per se do not indicate that the T-site most affected by dealumi-

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L.H. Ong et al. / Microporous and Mesoporous Materials 164 (2012) 9–20

Al(OH)3

OH2

H HO TO

O

O

TO

TO

Neutralization of BAS by EFAL TO

Al

[Si]

OH Al

OT

Al

[Si]

OT TO

O

O

TO

OT

OT

Scheme 4. Proposed structure for stabilization of framework Al by extra framework species.

Si O Al

O OH

Al

Al

O

O

Si

Scheme 5. Extra framework Al species in the form of Al(OH)2+ coordinated to two framework Al.

nation have close Al neighbors. However, combining the NMR results with the results of the cation exchange allows at least a tentative attribution of the species. The quantification of the signal intensity related to peak IV1 and peak IV2 shows concentrations of 90 and 33 lmol/g of Al, respectively for the zeolite prior to steaming. The ion exchange capacity determined by Na+ and Co2+ ion exchange leads to slightly lower concentrations, i.e., 84 lmol/ g of isolated and 18 lmol/g of close Al sites. Thus, we attribute the species giving rise to the NMR peak at 57 ppm to tetrahedral lattice aluminum with a T–O–T bond angle of 150°, while the one with a peak at 54.4 ppm is attributed to have a T–O–T angle of 155°. While we note that the latter occurs in close mutual vicinity, we cannot conclude whether the higher stability despite the higher bond angle result from the specific location of the sites or whether it is the preferential stabilization by extra lattice aluminum that enhances the thermal stability. We attribute the existence of such neighbored sites to the use of organic templates and the type of starting material in the synthesis process [32–34]. The formation of Si–O–Al bonds (as measured by the DG°rxn of condensation) is much more favorable than the formation of Si–O–Si bonds [35]. In addition, alumina has an overall higher solubility in basic medium than silica [36,37]. Given the higher overall solubility of alumina coupled with the more favorable formation of Si–O–Al bonds over Si–O–Si bonds, it is likely that these paired sites are incorporated in the crystals during the initial nucleation process in the inner core, while the isolated single tetrahedral aluminum atoms are located nearer to the pore entrance. Theoretical calculations have shown that for such paired Al sites to be stable, both Al atoms have to be located in the same channel in the Al–O–Si–O–Al sequence. The preference of this topological arrangement can be rationalized using the bond order conservation principle. The weaker Al–O bond makes the neighboring bonds relatively stronger [38]. The alternations of the bond weakening and strengthening make the second O–Al stronger which implies an attractive interaction between the neighboring Al (see Scheme 3). The stoichiometric neutralization of BAS by tetrahedral EFAl (peak IV3) suggests that all Al present in the species leading to the peak IV3 are associated with neutralizing the negative charge of framework Al. As also the highly distorted NMR invisible Al species

contribute to the neutralization of the negative charge of lattice aluminum we propose that the latter species is reversibly transformed to peak IV3. The more pronounced lability of Al with a lower T–O–T angle is somewhat surprising as one would generally assume these species to be more stable against dealumination due to the higher symmetry. However, the higher lability can be rationalized by taking into account that the polarizability is reduced for sites with more acute T–O–T angle, for which protonation is less favorable [35]. Theoretical calculations have indicated a high capacity of larger inter-tetrahedral angle to accommodate geometrical distortion arising from Si ? Al substitution and protonation [35]. Thus, we propose the flexibility of the larger T–O–T angle, which allows distortion of the Al–O bond especially in terms of the resulting angle strain to be the reason for their higher hydrothermal stability. They can also better accommodate distortions brought about by tetra coordinated EFAl, which helps to stabilize these framework Al against dealumination. Overall, our results indicate that the dealumination process starts with the breaking of the Al–O–Si bond of the lower T–O–T angle.

4.3. Slow dealumination step: potential influence of extra framework Al It has been argued above that Al located at sites of lower T–O–T angle of 150° is more labile and is removed preferentially. The extra framework Al formed during this dealumination step stabilizes the remaining Al against dealumination. In a formal way, this reflected in a net dealumination rate lowered close to zero, when the final concentration of sites protected by the interaction with cationic aluminum species is about 50 lmol/g. A simplified kinetic model considering only the forward dealumination reaction cannot reflect the latter findings. Hence, two more extensive models accounting for reversible dealumination have been explored, one sequential Al transformation mechanism (Scheme 1a) and one parallel Al transformation mechanism (Scheme 1b). In the former sequential mechanism the neutralizing EFAl species is not formed directly, but via the intermediate NMR invisible Al species. Following parameter optimization the description of the experimental data by the sequential reaction mechanism (Scheme 1a) is found marginally better than that for the parallel mechanism (Scheme 1b) judged from the residual values of the objective functions. Nevertheless, neither Scheme 1a nor Scheme 1b can be rejected within the precision of the experimental data. However, two observations concerning the numerical predictions are noteworthy. First, in the case of the sequential scheme all concentrations reach at more than 24 h steaming a quasi-steady state, while there is an ongoing formation of the Al species at 60 ppm (tetrahedral EFAl, peak IV3) predicted in the parallel scheme. Second, the applied constrained fitting leads for the parallel mecha-

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L.H. Ong et al. / Microporous and Mesoporous Materials 164 (2012) 9–20

H TO

O

TO

OH

+H 2 O

Al

[Si]

[Si]

Al TO

TO

OT

TO

H2 O

TO

OT

OT

OT

OT

OT

+ H2 O

TO

OH

OH [Si]

Al

TO

TO

OT

OH 2

HO

OT

OT

Si TO

OT

+ H2 O TO OH

HO

OH 2

[Si]

HO

Al TO

OT Si

TO

OT

OH HO

TO

OT

OT Si

TO

OT

+ H2O -

High T

OHTO

Al(OH)3

OT Si

TO

TO OH

HO

Si HO TO

OH

OT

OT Si

TO

OT Si

TO

OT

OT

Scheme 6. Proposed scheme for dealumination.

nism to a complete elimination of the reaction path from the Al species at 60 ppm to the Al species at 57 ppm (peak IV1, tetrahedral framework Al, T–O–T angle of 150°). As discussed above, we propose that the Brønsted acid sites carried by aluminum remaining in the framework are neutralized by extra framework Al to account for the difference of 27 lmol/g in tetrahedral aluminum that does not lead to Brønsted acid sites in the 24 h steamed sample. Once the more labile Al removed from the framework, it may adopt many forms including cations like Al3+, AlO+, Al(OH)2+,

Al(OH)2+ or neutral forms like AlO(OH), Al(OH)3, Al(OH)3H2O [32–39]. Benco et al. have simulated the largest diameter of the relaxed Al(OH)3H2O to be approximately 0.5 nm, which can still fit within the HZSM-5 pore channels. These extra framework Al species are most stable, when localized next to a Brønsted acid site [35]. In this process, the proton of the bridging hydroxyl group is transferred to the basic extra framework aluminum oxide cluster [36]. The proposed mechanism of neutralization of Brønsted acid sites by EFAl species is visualized in Scheme 4 where EFAl species are exemplified as Al(OH)3.

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L.H. Ong et al. / Microporous and Mesoporous Materials 164 (2012) 9–20

TO

Al(OH)3

Table 3 Proposed structure for intermediate and final Al species in the sequential dealumination of HZSM-5.

OT Si

TO

HO

HO

Si HO TO

OT

OT

OT

Acidic Al in tetrahedral coordination (80– 40 ppm)

Si

Si TO

Proposed structure of Al species Al species (chemical shifts in 27Al MAS NMR spectra)

OT

OH

TO

OT

TO

O

OT

TO Al(OH)3 OH HO

OT OH

Partially hydrolyzed Al species (NMR visible)

Si HO

OH

Al

HO OT

HO

Neutralized sites (single neighboring BAS) (60.5 + 0.5 ppm – tetracoordinated EFAl)

OT Si

Si

TO

OT

OH2 OH

Al

TO

O

TO Si

O

O

TO TO

Si Si

TO

OT Al

O

Neutralized Al at high density pockets (NMR visible) (60.5 + 0.5 ppm – tetracoordinated EFAl)

OT OT OT

O

TO

TO

OT Si

OT

OT -4H2O

OH OT

[Si]

TO

TO

OT OT

OT

TO

Al

TO

OH

HO

Si

OH

OT

Si

TO

OT

TO

TO

OT

OT Al

[Si]

Healing of defect sites by hydroxylated silicic species

TO

H

Si

O OH

Al

Al

O

O Al

Si

OT

OT

Si

O

OT TO

TO

Dealuminated Al from paired sitesa

Scheme 7. Scheme of the healing of defect sites.

Al3+, AlO+, Al(OH)2+, Al(OH)2+, AlO(OH), Al(OH)3, Al(OH)3H2O

Partially hydrolyzed Al Highly distorted Al–oxygen tetrahedra species (NMR invisible)b

Distribution Al species (%)

100

Octahedral Al species (10 ppm to 5 ppm)

framework Al forming BAS framework Al not forming BAS tetrahedral EFAl octahedral EFAl Al NMR invisible

80

OH2 HO

Al

HO

60

OH OH

OH2 a

As proposed in Refs. [34,36,37]. Deduced from the large quadrupolar coupling constant of these species which made them NMR invisible. Such species is in a much distorted environment with highly non-symmetric coordination.

40

b

20 0 0

4

8

12

16

20

24

Steaming time (h) Fig. 11. Graphical representation of the distribution of Al species during dealumination of the HZSM-5 framework. ‘‘Framework Al forming BAS’’ was obtained from IR spectroscopy of adsorbed Py; ‘‘framework Al not forming BAS’’ was calculated as difference of framework tetrahedral Al (sum of peak IV1 and peak IV2 in Al NMR spectra) and ‘‘framework Al imparting BAS’’; ‘‘tetrahedral EFAl’’ corresponds to peak IV3 in 27Al NMR spectra; ’’octahedral EFAl’’ was calculated from 27Al NMR spectra; ‘‘Al NMR invisible’’ was obtained as difference between the total amount of Al in the parent material and the sum of all above mentioned Al species.

DFT calculations [37] have also yielded stable minima for cationic extra framework Al species such as AlO+/Al(OH)2+ when coordinated to two Al in the framework as shown in Scheme 5. This may also help to account for the additional 9 lmol/g of non-acidic framework aluminum within the two T-sites as one extra

framework Al neutralized two Brønsted acid sites when adopting such coordination. 4.4. Elementary reaction steps proceeding during dealumination After the speciation of the aluminum atoms and acid sites in the zeolite structure, let us now discuss the sequence of the reaction steps involved in the dealumination process as depicted in Scheme 6. Dealumination starts with the breaking of the weakest Al–O bond rather than Si–O bond. This is due to the less electropositive nature of Al compared to Si, resulting in a weaker and longer Al–O bond (1.77–1.89 Å) compared to Si–O (1.67–1.72 Å) [40]. When the weakest Al–O bond is broken, a water molecule coordinates to the tri-coordinated Al by forming an Al–OH group. The additional proton formed from hydrolysis of water attaches to the remaining

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L.H. Ong et al. / Microporous and Mesoporous Materials 164 (2012) 9–20

Al–O–Si, but the rapid breaking of the remaining Al–O bonds results in its final state as part of the hydroxyl group attached to Si in the form of Si–OH. This process leads to the formation of new hydroxyl groups in the form of silanol nests (peak at 2.3 ppm in 1H MAS NMR spectrum) and also on partially hydrolyzed framework/extra framework Al (peak at 3.1 ppm in 1H MAS NMR spectrum). These partially hydrolyzed Al groups do not possess Brønsted acidity and the OH groups generated are of terminal nature (broad between 3700 and 3640 cm1. The complete removal of one Al should result in the formation of four Si–OH groups, but the relatively modest increase in the number of Si–OH groups forming silanol nests represented by the peak at 2.3 ppm in the 1H MAS NMR spectra, indicates rapid reorganization of the defect sites. This can occur through the migration of hydrolyzed silicic acid species formed by framework destruction or from the amorphous part of the zeolite filling the defect sites originated by Al elimination from the framework. Since XRD points to an almost unaffected crystallinity even after 24 h steaming, the latter reorganization should mainly involve the migration of silica species from the amorphous silica phase of the zeolite left from the synthesis process (Scheme 7). The Al speciation during dealumination of the HZSM-5 framework is shown in Fig. 11. The proposed structures of the different Al species are represented in Table 3.

5. Conclusions Two types of lattice aluminum oxygen tetrahedra have been identified in a high silica HZSM-5 (Si/Al 90) giving rise to 27Al MAS NMR peaks at 57 and 54.4 ppm as well as having T–O–T bond angles of 150° and 155°, respectively. When steamed at 450 °C the aluminum affiliated with the 57 ppm peak (surprisingly the one with the lower bond angle) decreased following a first order rate law. The parallel decrease of the band of the bridging SiOHAl groups at 3610 cm1 suggests that not only the visible extra lattice alumina generated in the process (27Al MAS NMR peaks at 60 ppm), but also an invisible fraction neutralizes lattice aluminum oxygen tetrahedra. The invisible and the (tetrahedrally coordinated visible one) are concluded to be kinetically connected. We speculate, therefore, that the invisible aluminum exists in the form of distorted tetrahedrally coordinated aluminum (hydr)oxo species. The process also gives rise to Al–OH groups at 3.1 ppm in the 1H MAS NMR spectra and contributes to the broad peak from 3726 to 3630 cm1 in the IR spectra. At higher steaming times, the concentration of bridging SiOHAl groups stabilized, while the concentration of isolated tetrahedral aluminum decreased further, albeit at a slower pace. This suggests that larger aluminum (hydr)oxo clusters must have been formed at a rate equal to the rate of dealumination. Such a process should eventually lead to the formation of octahedrally coordinated extra lattice aluminum. Indeed, the sample steamed for 24 h shows indications for such coordination in the 27Al MAS NMR spectrum. Surprisingly, the aluminum sites with a T–O–T bond angle of 155° were very stable under the conditions of steaming employed. It is interesting to note that the Co2+ exchange capacity was nearly equivalent to the concentration of these stable sites. While this can be a fortuitous coincidence, we would like to note that also the Co2+ ion exchange capacity did not vary with the steaming duration. Thus, we attribute the aluminum atoms to locations allowing to bridge with one Co2+ cation. It should be emphasized, however, that these sites are not located in the same five membered ring and do not show weaker acid strength than other SiOHAl groups. At the moment, we explore in detail whether these sites are preferentially stabilized by aluminum, or whether the aluminum tetrahedra are

more stable despite the more obtuse T–O–T angle. The present data show that dealumination of high silica HZSM-5 does not follow a random pattern, and that isolated SiOHAl groups are more labile than groups closer to each other. Acknowledgements Partial support in the framework of the Bavarian regional project on new materials NW-0711-0009 is gratefully acknowledged.

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