Titanium boralites with MFI structure characterized using XRD, IR, UV-Vis XANES and MAS-NMR techniques

Zeolites: A Refined Tool for Designing Catalytic Sites L. Bonneviot and S. Kaliaguine (editors) 9 1995 Elsevier Science B.V. All rights reserved.

535

T i t a n i u m boralites with M F I structure characterized using X R D , IR, U V - V i s X A N E S and M A S - N M R techniques D. Trong Ona, b, M. P. Kapoorb, S. Kaliaguine b, L. Bonneviot a and Z. Gabelica c Department of aChemistry and bChemical Engineering and CERPIC, Laval University, SteFoy, G1K 7P4, Canada, CDepartment of Chemistry, Laboratory of Catalysis, Facult6s Universitaires de Namur, B-5000 Namur, Belgium Two series of boralites and titanium boralites of MFI structure have been synthesized. The structural state of titanium and boron in the silicalite framework has been investigated using XRD, IR, UV-Vis, XANES and MAS-NMR spectroscopic techniques. The quantitative determination of framework titanium and tetrahedral framework boron was made by XANES at the Ti K-edge and liB MAS-NMR. It was shown that both titanium and boron can be incorporated in the framework. In spite of the formation of extra-framework titanium in presence of boron, even at low Ti/Si ratio, the maximum of Ti incorporated seems not affected by boron and optimized by an excess of H202 during the gelification. In calcined samples, the concentration of tetrahedral framework boron is smaller for higher framework titanium content. 1. INTRODUCTION The discovery of crystalline titanium silicalites with MFI or MEL structures, TS-1 and TS-2 respectively, has extended the use of molecular sieves to the catalysts of oxidation reactions [ 1,2]. The simultaneous incorporation of a trivalent metal ions (e.g., B 3+, A13+, Ga 3+, or Fe 3+) along with Ti 4+ in MFI, MEL [3-5], and beta [6] structures has been reported. It may be anticipated that such solids are active both in oxidation reactions like titanium silicalites and in acid-catalysed reactions like aluminosilicates [3,4]. Recently, it has been demontrated that the epoxydation reaction occuring on framework titanium is followed by an hydrolysis of the epoxide into ot-diols in the presence of a mild acidity [7]. Such a bifunctional catalyst is obtained with a zeolite containing titanium and aluminium. Since in a silicalite containing boron, designated as boralite, framework boron is known to introduce milder acidic bridging hydroxyl groups, B-(OH)-Si, than aluminium, this element has a better potential for such reaction. Nevertheless, both boron and titanium have to be incorporated in the framework to yield the proper bifunctionnality. 2. EXPERIMENTAL Two series of boralites (BS-1) and titanium boralites (TBS-1) catalysts were prepared from a gel containing TEOS, TEOT, H3BO3, H202, H20 and the appropriate template by a method previously described [8]. The gel composition was: 0.40TPAOH- xTiO2-0.1 B203-SiO235H20-yH202 where x =0.00-0.03 and y = 0.04 - 0.4). The gel was charged in an autoclave and maintained in hydrothermal conditions for five days at 488 K. The solids were calcined under continuous flow of dry ammonia, or oxygen gas at 450~ and 550~ respectively. Sodium exchange was carried out by stirring 0.5 g of molecular sieve in 30 ml of 1M aqueous sodium bromide during 24 h at room temperature. For deboronation of calcined

536 samples, an aqueous hydrochloric acid (pH= 1) was used at room temperature. The elemental analysis was performed by flame atomic absorption spectroscopy for titanium and sodium, and induced coupled plasma atomic emission spectroscopy for boron analysis. The X-ray diffraction patterns of the samples were recorded on a Rigaku D-MAX II VC X-ray diffractometer using nickel filtered Cu K0~ radiation. The unit cell volumes were calculated from XRD using silicon as an internal standard. The Ti K-edge X-ray absorption spectra were collected at the radiation synchrotron facility of the LURE (France). The white radiation Was monochromatized by a Si (111) two-crystal monochromator. The normalized XANES spectra were analyzed using a classical edge normalization procedure and energy calibration to the first peak of the K-edge of a titanium foil [9]. Dehydrated samples were obtained by evacuation at 573 K in vacuum and transferred under dry argon into a vacuum tight cell for measurements. Diffuse reflectance UV-Vis spectroscopy was performed using a Perkin-Elmer Lambda 5 spectrophotometer using MgO as a standard. IR spectra were recorded with a Bomen 102 FTIR spectrometer on self supporting pellets of samples mixed with KBr. 11B_MAS NMR spectra were obtained on a Bruker CXP-300 spectrometer by a "one-cycle" type measurement: resonance frequency: 128.288 MHz, r.f. field: 9.4 G (10 ~ pulses); repetition time: 0.2s, sweep width: 62.5 kHz, with 4K data points. Typically 2000 free induction decays were accumulated per sample. MAS was 10 kHz, using a 4 mm zirconia rotor. The chemical shifts were determined from BF3.OEt2, used as an external reference.

3. RESULTS AND DISCUSSION XRD: The X-ray pattern of the silicalite revealed a Pnma-orthorhombic symmetry from which the indexation was performed. The 2.9% boron leads to a decrease of the unit cell volume from 5335 to 5280 s consistently with previous reports (Table 1) [10]. On the other hand, the [3.4]TS-1 exhibits a unit cell expansion of 35 ~3 in agreement with literature data [ 11 ]. This observation is logical on account of the differences in the M-O distances (1.79, 1.61 and 1.47 .~ for M=Ti 4+, Si4+ and B 3+ respectively) and thereby supports the incorporation of Ti 4+ and B 3+ in the framework lattice. A mixte effect is observed for the titanium , 1 ' ' ' ' 1 .... I .... I .... I .... I .... I ' " ' ' ~ ] boralites for which unit cells are 5400 larger than pure boralite, 9 a 9 [2.9]BS and smaller than pure eq silicalite (Table 1). Taking the 5360 b , boralite as reference, it is clear that the unit cell volume progressively expends when the o 5320 ;> titanium loading increases at constant boron concentration. A 5280 linear fit of the expansion can be I ~ J l ~ l ~ , , , I , , , , I , ~ , , I .... I , . , , ! .... proposed as in the case of TS-1 0 1 2 3 (Fig. 1). The slopes (dV/dx) T~Si in molar% associated to the unit cell expansion is smaller for TBS Figure 1. Evolution of the unit cell volume with the titanium than TS (15 instead of 20 A3, content for, a) TS (A this work, 9 ref. 11), b) TBS versus respectively)[ 11 ]. Further framework titanium obtained from XANES spectra and c) investigation is indeed necessary TBS versus total content of titanium. as this expansion must be correlated to the effective level of Ti incorporation.

537

Table 1 Chemical composition of crystalline solids and XANES, MAS-NMR and XRD characteristics for titaniumboralites and boralites.

zeolite

XANES

NMR

tetrahedral Ti b

tetrahedral B c

hydrated

dehydrated

T/(O+T) (%)

T/(O+T) Ti/Si (%) (%)

a [ 1.5-3.0]TBS-A [1.5-2.9]TBS-C e [ 1.4-2.7]TBS-A [ 1.4-2.7]TBS-C f [2.3-2.7]TBS-A [2.2-2.7]TBS-C [3.2-2.8]TBS-A [3.4-2.7]TBS-C [3.3-3.8]TBS-A [3.3-3.6]TBS-C [3.4]TS-C [3.0]BS-C [2.9]BS-C silicalite-C [1.5]TS-C

20 . 25 25 30 . 60

35 .

.

.

. 100

B/u.c.

2.8 1.6

. 1.5

XRD

Ti/Si B/Si (%) (%)

unit cell d volume (A3)

1.5 1.5 1.4 1.4 2.3 2.2 3.2 3.4 3.3 3.3 3.4 -

3.0 2.9 2.7 2.7 2.7 2.7 2.8 2.7 3.8 3.6 3.0 2.9

5305

1.5

-

line (-3.7 ppm)

0.5 . 0.8 0.9 1.5 1.7 -

55 40 45 50 -

chemical analysis

2.3 0.7 2.1 0.4 2.4 0.3 2.8 2.1 . -

.

5325 5325 5330 5370 5280 5335 5375

a) 1.5 and 3.0% for Ti/Si and B/Si atomic ratio respectively in as-synthesized or calcined TiBoralite are referred to as [ 1.5-3.0]TBS-A or C respectively, b) percentage of tetrahedral (T) and octahedral (O) titanium calculated from XANES results, c) content of tetrahedral boron calculated from NMR results, d) calculated from XRD, e) and f) H202/B=0.3 and 1.0 in the gel respectively.

Framework IR: The IR spectra o f the boralite and titanium boralite samples with different treatments are given in Figure 2. In all cases, the IR spectra w e r e typical of pentasil zeolites. T h e well defined IR bands at 800 and 455 c m -1 and the saturated region 1000-1300 c m -1 are characteristic of SiO4 tetrahedra, while the vibrational band at 555 cm-1 confirms the presence of five m e m b e r tings of the pentasil structures. The IR band at 920 cm-1 can be assigned to the presence o f the tetracoordinated f r a m e w o r k boron and it is o b s e r v e d in all b o r o n pentasils. H o w e v e r , for the calcined sample (Fig.2A), a strong band is also appearing at 1380 c m -1 which can be assigned to tricoordinated f r a m e w o r k boron [ 12]. The transformation o f the absorption band at 920 c m -1 to 1380 c m -1 upon calcination is logically assigned to the change o f b o r o n coordination from tetrahedral to trigonal. On the other hand, after the N H 3 treatments (Fig. 2Ab), f o l l o w e d by N a +- e x c h a n g e (Fig. 2A-c) and calcination (Fig. l d), the IR spectra do not s h o w the characteristic BO3 absorption at 1380 c m -1. It is c o n c l u d e d that tetrahedral f r a m e w o r k b o r o n is stable w h e n N H 4 + or N a + ions are c o u n t e r b a l a n c i n g the f r a m e w o r k negative charge, and unstable when the counterion is the proton (Fig. 2A-e)[ 12]. The IR spectra o f the titanium boralites are similar to those of boralite samples (Fig. 2B). A

538 new band appears at 960 cm-1 and corresponds to the fingerprint of titanium incorporation in boron free silicalites [ 13]. The presence of IR bands at -920, 960 and 1380 cm-1 in all samples strongly suggests the simultaneous incorporation of Ti and B in the silicalite framework.

4-

NMR: Assuming that all b o r o n atoms are N M R A 1380 visible. The M A S - N M R method can be used for a 1800 1500 1200 900 600 300 1100 900 700 quantitative determination of -1 -1 cm cm the amount of boron actually substituted for Si in zeolite Figure 2: IR spectra of (A) the boralite, [4.0]BS and (B) titanium frameworks, whereas boralite, [3.4-4.0]TBS samples: a) as-synthesized, b) after NH3 chemical analysis can only treatment at 450~ for 4h, c) sample (b) exchanged with 1M NaBr provide a bulk boron solution at ambient temperature, d) sample (c) calcined at 550~ in air concentration. The NMR line for 6h, e) sample (a) calcined at 550~ in air for 6h. located at 5 - - 3 . 7 ppm is unambiguously assigned to BO4- tetrahedra in a crystalline boralite structure. This line is very sensitive to the local structure and the electronic environment [ 14,15]. A quantitative determination of tetrahedral boron content in the samples can thus be estimated by measuring the intensity of NMR line at -3.7 ppm. The 11B NMR spectrum of as-synthesized [3.0]BS sample exhibits a single narrow peak at --4.0 ppm (Fig. 3A), consistent with most of the boron ions incorporated as tetrahedral BO4units in the framework. Two very broad lines of much weaker intensity around -2 ppm correspond to framework linked trigonal boron [15]. Upon calcination, the intensity of tetrahedral boron line is decreased and is slightly shifted to -3.7 ppm. The trigonal framework lines are relatively increased in addition a hump assigned to an extra-framework boron arises at - 6 ppm. The observed small shift for the tetrahedral boron can be explained by a change in the chemical environment and a sharp decrease of the dipolar interaction between boron and other nuclei upon calcination. The 11B NMR spectra of the as-synthesized and calcined TBS samples containing both Ti and B are essentially similar to those of the corresponding boralite samples (Fig. 3B). The intensity of the tetrahedral boron line at - -3.7 ppm for the calcined TBS sample is found to be decreased while the intensity of the framework and extra-framework trigonal lines are clearly enhanced as compared to calcined BS sample of same boron concentration. The NMR quantification of boron by 11B NMR yields less boron than the amount measured by chemical analysis for several of the samples, especially for those calcined at 550~ (Table 1). It seems that the amount of framework and extra-framework trigonal boron at - -2 and - 6 ppm are not totally detected by 11B NMR, because the NMR lines are highly asymmetric species and may be broadened beyond the detection limit. Therefore, the presence of observed NMR lines accounts only qualitatively for the existence of framework and extra-framework trigonal boron. UV-Vis spectroscopy. The pure boralite exhibits no UV band in the 200-400 nm range where lie the features of TBS and TS samples which indeed for the two materials are very

539 0.5 -3.6

:j

%

J 8 6 4 2 0 -2 -4 -6 -8 PPM

0 -3.6

500

I

I

I

I

I

I

450

400

350

300

250

200

nlrl ~ +6

-I.

9

i

.

i

.

i

.

i

,

i

,

i

,

i

b

Figure 4. UV-Visible diffuse reflectance spectra of titanium boralites with the same Ti and B loadings of 3.0 and 1.5 atom % respectively and varying H202/B ratio in the gel: a) [1.5-3.0]TBS (H202/B-0.3), b) [1.4-2.7]TBS (H202/B=l.0) and c) [1.5]TS.

I

similar. Between 270 to 400 nm, the absorption exhibits a broad peak assigned to TiO2 anatase, while below 270 nm, an intense peak is assigned to isolated Figure 3. l I B MAS-NMR titanium species incorporated in the framework [16]. spectra of (A) boralite, [3.0]BS The UV-Vis spectra of TBS samples synthesized in the and (B) titanium boralite, [3.4presence of boron show a framework titanium band at 210 nm along with the extra-framework band of varying 2.7]TBS: a) as-synthesized b) intensifies. The series of titanium boralites prepared calcined samples. with varying H202/B ratio in the gel (Fig. 4). This is apparently consistent with a diminution of extraframework Ti concentration when an excess of hydrogen peroxide is added. The UV band of extra-framework titanium is blue shifted (at 325 and 340 nm for samples (a) and (b) respectively in Fig. 4) in comparison with the position expected for bulk TiO2 (at 350 nm). This blue shift clearly indicates a change in the nature of extra- framework species. In addition, the titanium loading chosen for these samples is low enough to avoid the formation of extraframework Ti species in the boron free TS sample as indicated by the absence of I.W bands in the 270-400 nm range for [1.5iTS (Fig. 4). Conversely, the systematic presence of a UV band in this region for TBS clearly shows that extra-framework Ti species are systematically generated in presence of boron. The hydrogen peroxide concentration seems to be an important parameter influencing the extra-framework species nature and concentration. Thus it is concluded that framework incorporated Yi increases in concentration as the hydrogen peroxide concentration increases. By contrast, the inconsistent intensity variation of the peak in the 270400 nm range versus titanium loading indicates that no quantification of this species should be attempted by UV spectroscopies. 8 6 4 2 0 -2 -4 -6 -8 PPM

XANES: The Ti K-edges of dehydrated and hydrated TBS-1 were compared to those of bulky TiO2 anatase and dehydrated [1.5iTS-1. The pre-edges of TBS samples exhibit intermediate features compared to the two reference materials (Fig. 5). This suggests the presence of a mixture of sites in the TBS samples. Taking into account the absence of anatase

540 or any form of extra-framework Ti in the [1.5]TS, the Ti K-edge of this sample 2 was taken as a reference for 100% o incorporation of titanium in the ~. 1.6 framework. O .~ 1.2 Accordingly, the Ti K-edge of anatase was adopted as reference for octahedral bl 0.8 titanium. Simulations of the titanium Kedges using a linear combination of the 0.4 two reference edges led to excellent fits of O z the dehydrated TBS- l(Fig. 5, doted o lines). A quantitative determination of tetrahedral and octahedral titanium -0.4 distribution in TBS-1 samples is 4960 4980 5000 5020 5040 proposed on this basis [9] (Table 1). Energy/eV For dehydrated boralites prepared with various titanium loadings (1.5, 2.2 and Figure 5. Ti-K-edge XANES spectra of: a) anatase, b) 3.4 %, see Table 1), one observes that [I.5-3.0]TBS, c) [1.4-2.7]TBS and d) [1.5]TS; the higher the titanium loading the higher Linear combinationfit (---) of XANES spectra using as the incorporation in tetrahedral site (35, reference spectra those of anatase for octahedral 40, 45 %). In the series where the hydrogen peroxide concentration was coordination (a) and [1.5ITS for tetrahedral varied at a constant Ti loading, from 3 5% coordination (d). up to 55% of tetrahedral titanium was found for samples made from gels 3.5 containing a H202/B higher than 1 (Table 3 1). According to the XANES results, the maximum titanium incorporated in ~ 2.5 iiiiiiiiiiiiiiiiiii iiiiiii presence of boron is about 1.5%. For hydrated samples, the fit quality was not ~ 2 as good as for dehydrated samples and ~ 1.5 ........i...........i..................!....................!.....................!.................... the confidence on site occupancy was not 1 as reliable. This is attributed to a change :: :: .i :: of symmetry more likely due to the 0.5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . addition of a water molecule in the 0 coordination sphere of Ti [ 17,18]. -0.5 0 0.5 1 1.5 The quantitative data on tetrahedral Ti/Si (%) framework or total boron and framework or total titanium contents in titanium Figure 6: Total boron content obtained by chemical boralites obtained by using XANES, analysis (curve a), and tetrahedral boron content NMR, and chemical analysis (Table 1) are gathered in Fig. 6 to facilitate the obtained from liB MAS-NMR intensities for asdiscussion for the series of assynthesized (curve b) and calcined (curve c) boralite and synthesized (Fig. 6b) and calcined 9 titanium boralite samples samples (FI~. 6c). The slope d(B/Si)/d(Ti/Si) is slightly lower tbr curve (b) than for curve (a) and much lower for curve (c). This strongly suggests that during calcination more boron is converted into trigonal and extra-framework species in samples with higher framework titanium content. The tetrahedral framework boron is decreased from 2.1% for the boralite sample to -~0.4% for the titanium boralite sample as the framework titanium increased from 0% to 1.6%. Finally, it is quite clear that boron hinders the incorporation of titanium into framework during crystallization whereas titanium favors the extraction of boron out from the framework during calcination. ,

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N

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i

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,

,

i

...............

,

,

,

i

i

,

,

. . . . . .

j

i

....

a

...........

541 4. CONCLUSION The combined XRD, IR, UV-Vis, XANES and MAS-NMR investigations on the structural state of titanium and boron in the silicalite of MFI structure led to the following conclusive points: i) both titanium and boron can be simultaneously incorporated in the MFI-framework, ii) the presence of boron leads to some extra-framework titanium formation even at low Ti/Si ratio but does not affect significantly the limit of titanium incorporation, iii) high hydrogen peroxide concentrations were necessary to optimise the incorporation of Ti in the framework, iv) less tetrahedral framework boron was left behind for high framework titanium contents during calcination. REFERENCES

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3. 4.

,

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8.

.

10. 11. 12. 13. 14. 15. 16. 17. 18.

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