[acs symposium series] zeolite synthesis volume 398 || hydrothermal isomorphous substitution of...

Chapter 27

Hydrothermal Isomorphous Substitution of Boron in Zeolite ZSM-5/Silicalite

Bogdan Sulikowski1 and Jacek Klinowski

Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, England

We report (i) isomorphous substitution of boron, by secondary synthesis, into silicalite and into highly siliceous (Si/Al>400) ZSM-5; and (ii) an improved direct synthesis of zeolite (Si,B) -ZSM-5. The chemical status of Β in the boronated products depends upon reaction conditions. Careful control of the concentration of the base, the borate species and of the duration of treatment, allows the preparation of samples containing only 4-coordinated Β or a mixture of 3- and 4-coordinated Β in various relative concentrations. The products were characterized by magic-angle-spinning (MAS) NMR and infrared (IR) spectroscopies and by powder x-ray diffraction (XRD).

Elements such as B, Ga, Ρ and Ge can substitute for Si and Al in zeolitic frameworks. In naturally-occurring borosilicates Β is usually present in trigonal coordination, but four-coordinated (tetrahedral) Β is found in some minerals and in synthetic boro- and boroaluminosilicates. Boron can be incorporated into zeolitic frameworks during synthesis, provided that the concentration of aluminium species, favoured by the solid, is very low. (B,Si)-zeolites cannot be prepared from synthesis mixtures which are rich in aluminium. Protonic forms of borosilicate zeolites are less acidic than their aluminosilicate counterparts (1-4), but are active in catalyzing a variety of organic reactions, such as cracking, isomerization of xylene, dealkylation of arylbenzenes, alkylation and disproportionation of toluene and the conversion of methanol to hydrocarbons (5-11). It is now clear that the catalytic activity of borosilicates is actually due to traces of aluminium in the framework (6). However, controlled substitution of boron allows fine tuning of channel apertures and is useful for shape-selective sorption and catalysis.

1On leave from the Institute of Organic Chemistry and Technology, Krakow Technical University, Poland

0097-6156/89A)398-0393$06.00/0 ο 1989 American Chemical Society

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In Zeolite Synthesis; Occelli, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

394 ZEOLITE SYNTHESIS

We have earlier addressed the problem of the post-synthesis insertion of aluminium in zeolites ZSM-5 (12) and Y (Hamdan, H.; Sulikowski, B.; Klinowski, J. T.Phys.Chem., (in press)). The substitution of gallium in silicalite-Π has also been achieved (13). It was therefore of considerable interest to establish whether boron can also be incorporated into silicate frameworks after the completion of synthesis. We report isomorphous substitution of boron into zeolite ZSM-5 by mild hydrothermal treatment with borate species.

Experimental

Materials. The parent zeolite (Si,Al)-ZSM-5 was synthesized according to Argauer and Landolt (14), calcined in air at 500°C, and subjected to two-fold ion exchange with aqueous 1M N H 4 C I . The 86% ammonium-exchanged zeolite was washed and dried. It had Si/Al=47.6 (by wet chemical analysis). Dealuminated ZSM-5 was prepared by two-fold steaming of a NH4 -ZSM-5 at 540°C for 2 h each time followed by extraction of aluminium with 2M HC1 under reflux, washing and drying. The product (De-ZSM-5) had Si/Al>400.

Silicalite was synthesized according to Grose and Flanigen (15), using tetrapropylammonium hydroxide and Ludox AS-40 as a source of silicon. The product was washed, dried and calcined in air at 600°C giving crystalline silicalite with Si/Al=563 as determined by x-ray fluorescence (XRF) and atomic absorption (AA).

Starting materials for the direct synthesis of (Si,B)-ZSM-5 were fumed silica (Cabosil), tetrapropylammonium bromide (TPA-Br), ammonium fluoride and orthoboric acid (16). The source of silica was mixed with TPA-Br and water and then a mixed solution of N H 4 F and H 3 B O 3 was added under vigorous stirring. The resultant gel was homogenized for 1.5 h and transferred into a Teflon-lined stainless steel autoclave which was then heated at 200°C for 17 days. Zeolite crystals were washed, dried at 60°C and hydrated in a desiccator.

Hydrothermal treatment with borate species. The boronation procedure was as follows: first, solution A was prepared by dissolving 7.75 g of B2O3 in 100 ml K O H (pH«8). The n B NMR spectrum of this solution (not shown) consists of one signal at 19.9 ppm (from BF3 · Et20), corresponding to trigonal boron (17). 1.5 g of the zeolite was then placed in a polypropylene bottle and 20 ml of solution A added (zeolite/B2C>3 = 0.9) and the final pH of the suspension was adjusted with additional amounts of KOH. The bottles were tightly capped and kept in an oven at the desired temperature for 24-42 h without stirring. The products were thoroughly washed with distilled water, dried at 80°C and hydrated in a desiccator. The conditions of the hydrothermal treatment are summarized in Table I.

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27. SULIKOWSKI AND KLINOWSKI Substitution of Boron 395

Table I. Conditions of hydrothermal treatment of dealuminated ZSM-5 (samples 1-2) and silicalite (samples 3-9), chemical shift (CS) and

full-width-at-half-maximum (FWHM) of the n B M A S N M R signals

Sample (g) B2O3 K O H cone. Temp. T i m e » B M A S N M R (g) (M) (°C) (h) CS (ppmt) F W H M (Hz)

1 0.5» 0.20 0.08 80 24 -3.97 93 2 0.2* 0.43 0.01 80 40 -3.84 91 3 1.5 1.55 <0.01 80 24 -3.97 115 4 1.5 1.55 0.03 80 24 -3.96 109 5 1.5 1.55 0.05 80 24 -3.92 104 6 1.5 1.55 0.16 80 42 -3.94 92 7 1.5 1.55 0.10 80 42 -3.93 95 8 1.5 1.55 0.50 28 25 - -9 1.5 1.55 2.00 28 25 - -

* dealuminated ZSM-5 (Si/Al>400). t from external BF3«Et20.

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Magic-angle-spinning NMR. n B MAS NMR spectra were measured at 128.33 MHz using a Bruker MSL-400 multinuclear spectrometer. Samples were spun in Vespel rotors at 4-5 kHz using air as the spinning gas. Radio-frequency pulses of 2 μ8 duration were applied with 300 ms recycle delay. Short pulses are necessary if quantitative spectra of quadrupolar nuclei are to be obtained (18). From 6000 to 24000 transients were acquired for each spectrum, and all spectra were recorded under the same conditions. n B chemical shifts are quoted in ppm from external BF3 · Et20. A calibration curve was prepared using known amounts of the as-prepared (Si,B)-ZSM-5. The samples were also analyzed by AA. The samples were hydrated in the desiccator over saturated N H 4 N O 3 solution prior to measurement.

X-ray diffraction. XRD patterns were acquired on a Philips PW1710 vertical goniometer using CuKa radiation selected by a graphite monochromator in the diffracted beam. All the samples were fully hydrated before XRD diffractograms were measured. Silicon powder was used as an internal standard.

Infrared spectra. IR absorption spectra in the framework vibration region (400-1400 cm"1, resolution 1 cm-1) were obtained with a Nicolet MX-1 Fourier transform spectrometer using the KBr pellet technique.

Results and Discussion

Silicalite (samples 3-9) and dealuminated ZSM-5 (samples 1-2) were subjected to hydrothermal treatment under mild alkaline conditions at various boron-to-zeolite ratios, temperatures and times of treatment (see Table I). To consider the question of the status of boron in the boronated samples we have used 1 1 Β MAS NMR, a technique capable of providing direct information on the type of short-range environment of the nucleus.

Typical n B MAS NMR spectra of boronated samples 1 and 7 are shown in Figure 1. The spectrum of boronated sample 1 prepared from dealuminated ZSM-5 consists of one narrow signal centred at -3.97 ppm. The spectrum of boronated silicalite contains a second broad signal [see Figure 1(b)]. We shall address first the question of the origin of the sharp 1 1 B NMR signals with the chemical shift at about -3.9 ppm (Table I). In borosilicates boron can adopt either three- or four-fold coordination. In naturally-occurring borosilicates it is usually present in trigonal coordination, but four-coordinated (tetrahedral) Β is found in some minerals (such as datolite, garrelsite and reedmergnerite, the boron analogue of albite (22)) and in synthetic boro- and boroaluminosilicates. Tetrahedral boron in the boro- and boroaluminosilicates gives rise to a sharp signal with a chemical shift in the range of -3.3 to -4.0 ppm from BF3 · Et2U (2,4,23-25). In minerals such as kernite and inderite, BO3 and BO4 groupings can be unambigously distinguished by NMR. Moreover, relative spectral intensities were in excellent agreement with both x-ray

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- 3 . 9 p p m

I

te t rahedra l

— I 1 ι Ο - 5 - 1 0

ppm from B F 3 » E t 2 0

Figure 1. 1 1 LB MAS NMR of boronated samples in the absolute intensity mode, (a) sample 1; (b) sample 7.

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studies and chemical composition (26). Accordingly, the sharp signal at about -3.9 ppm in the boronated samples is assigned to tetrahedrally coordinated framework boron.

In order to check this conclusion further, a number of (Si,B)-ZSM-5 zeolites were prepared via direct synthesis. The crystals of B-ZSM-5 are all very well developed with the length of about 200 μπι (Figure 2). In the as-synthesized (Si,B)-ZSM-5 the chemical shift of boron is -3.95 ppm with FWMH=94 Hz [Figure 3 (b)]. The signal is narrow and symmetric because of small quadrupole interactions of tetrahedral n B . The signals at -3.9 ppm in the spectra of boronated samples are also narrow (91 to 115 Hz, see Table I and Figure 1) and are clearly due to tetrahedrally coordinated framework boron.

The assignment of the second broad signal in Figure 1 (b) for sample 7 is less straightforward. One possibility is that it corresponds to three-coordinated boron. Trigonal boron is usually observed in dehydrated (Si,B)-ZSM-5 (2,4). Upon dehydration of the borosilicates with ZSM-5 structure containing framework tetrahedral boron atoms, the coordination of boron changes to trigonal and in completely dehydrated (Si,B)-ZSM-5 zeolites nearly all the boron exists as BO3 units, although small amounts of tetrahedral sites were reported by Kessler et al. even in fully dehydrated samples (16). The 1 1 B spectrum of a dehydrated (Si,B)-ZSM-5 shows a typical quadrupolar pattern composed of two distinct peaks (both corresponding to the same kind of boron) with a centre of gravity at approximately -5 ppm. In partially dehydrated zeolites the quadrupolar pattern of trigonal boron is superposed onto a symmetric signal from tetra-hedrally coordinated boron. Moreover, upon complete rehydratation of borosilicate only one signal was always observed thus indicating that the lower symmetry units BO3 transform fully to BO4 groupings (2,4). Since our samples 1-9 were completely hydrated before the NMR spectra were recorded, but the broad signal is still present in most samples, we conclude that it comes from non-framework BO3 units in amorphous species present within the zeolite channels. The broad signal was also found in the spectrum of Pyrex glass [see Figure 3 (a) and refs. 4, 23 and 26] and in the spectra of a variety of boron minerals (26).

Table 1 1 gives the absolute intensities of NMR signals and the concentration of tetrahedral and trigonal boron sites. The boron content of the samples was calculated from spectral intensities. It follows that the amounts of boron introduced into silicalite/ZSM-5 during hydrothermal treatment are relatively small, between 0.17-0.50 Β atoms per unit cell. Of this amount up to 0.36 B/u.c. was found in tetrahedral coordination (sample 6, treated with 0.16M KOH for 42 h). There is also a systematic decrease of the (tetrahedral) boron line width from 115 to 91 kHz (Table I) with the increase alkalinity of the treatment.

Temperature of treatment is also important. The optimal temperature for the reinsertion of aluminium into ultrastable zeolite Y is 60-80°C (Hamdan, H.; Sulikowski, B.; Klinowski, J. T.Phys.Chem., (in

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Figure 2. Optical microscope photograph of (Si,B)-ZSM-5 prepared by direct synthesis (14) and containing 2.2 boron atoms per unit cell. The crystals are remarkably well developed and uniform in size (ca. 200μπι long), but are all intimately twinned.

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20 10 0 -10

ppm from BF 3.Et 20

Figure 3. MAS NMR spectra (not in the absolute intensity mode), (a) Pyrex glass; (b) as-prepared (Si,B)-ZSM-5 (2.2 boron atoms per unit cell).

Table Π. Relative 1 1 B spectral intensities

Sample Relative total Tetrahedral Trigonal Boron atoms per intensity boron boron unit cell

(%) (%) (tetrahedral) (total)

1 77.4 100.0 - 0.17 0.17 2 94.8 75.9 24.1 0.16 0.21 3 177.1 47.8 52.2 0.19 0.40 4 184.0 40.7 59.3 0.17 0.41 5 192.6 38.7 61.3 0.17 0.43 6 222.3 72.7 27.3 0.36 0.50 7 209.7 65.1 34.9 0.31 0.47

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press)). However, only traces of boron are present in silicalite treated at room temperature (samples 8 and 9), even though the alkalinity of the boron-ating solution was considerably higher than for other samples. Although unit cell parameters are a linear function of Si/Al (27,28,30) and Si/B ratios (29), the amounts of tetrahedral Β are too small to be quantitatively monitored by XRD.

The position of the asymmetric stretch T-OT vibration in infrared spectra of the framework vibration region (at about 1100 cm-1) is a sensitive probe of Si/Al and Si/B ratios in aluminosilicates (27) or borosilicates (31). Infrared spectra of the boronated samples revealed a slight shift of the asymmetric stretch silicalite band at 1100 cm -1 to about 1098 cm -1, consistent with the boron content calculated from NMR line intensities (Jansen et al. (31) reported a 10 cm-1 shift upon substitution of 4.1 boron atoms per unit cell of ZSM-5).

The crystallinity of the boronated samples was monitored by XRD. No changes in line intensities were generally observed, which indicates that crystallinity of samples is fully maintained during secondary synthesis. The framework remained intact even when silicalite was treated with concentrated 2M KOH (sample 9). Alkaline treatment with a strong base did, however, induce a change of symmetry. Both the starting silicalite and the sample treated with 0.5M KOH were monoclinic, while sample 9 (treated with 2M KOH) was orthorhombic (see Figure 4). Pure silicalite has 24 crystallographically non-equivalent sites for Si and is monoclinic at room temperature (19-21) (Williams, J.H.; Axon, S.A.; Klinowski,}. Chem. Phys. Lett, (submitted)). It undergoes a transition to an orthorhombic form (with 12 crystallographically non-equivalent sites for Si) at 82°C (20). In ZSM-5, which is nominally isostructural with silicalite, the temperature,^ at which the transition occurs depends on the level of "impurities" (i.e. species other than silica, such as the residual aluminium in the framework, cations, adsorbed water and organics). ZSM-5 with relatively high Al contents is orthorhombic at room temperature because the transition occurs at Tt<20°C. The sample of silicalite treated with 2M (but not with 0.5M) base solution was orthorhombic, and it is clear that treatment with a strong base introduces sufficient amount of "impurities" for the phase transition to take place below the room temperature.

The disparity in size of the aluminate and the silicate tetrahedra must be the reason why, at least for some frameworks, the range of Si/Al ratios, and therefore the extent of the post-synthesis isomorphous substitution of Al for Si is limited (27). For boron, with the ionic radius of 0.23 Â as compared with 0.51 Â for aluminium, the disparity in size is even greater (2). Quantum chemical calculations predict that the tetrahedral coordination of aluminium is favoured in comparison with BO4 groupings (32,33). An attempt to insert boron into the framework of ferrierite (34), a structurally related zeolite, was unsuccessful.

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133

I I ι ι J J

(a) 1 3 8 13 16 23 28 33 38 43 48

2Θ (degrees)

Figure 4. XRD patterns, (a) Parent silicalite; (b) after treatment with borate solution in 0.5M KOH (sample 8); (c) after treatment with borate solution in 2M KOH (sample 9). Arrows indicate peaks which split upon the orthorhombic/monoclinic phase transition.

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Acknowledgment

We are grateful to Unilever Research, Port Sunlight, for support.

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