Synthesis of iron-silicate analogs of zeolite beta

Synthesis of iron-silicate analogs of zeolite beta R. Kumar, A. Thangaraj, R.N. Bhat, and P. Ratnasamy National Chemical Laboratory, Pune, India Crystalline beta zeolites containing iron have been prepared and characterized. Evidence for the presence of Fe in the lattice framework is obtained from chemical analysis, XRD, framework i.r., e.s.r., and solid-state MAS n.m.r, spectroscopies, d.t.a/t.g., magnetic susceptibility, and catalytic activity data. Chemical analysis and n.m.r, confirm the absence of a significant amount of AI in the zeolite. (SIO2/AI203 > 2000). The sample was white in color. Lattice Fe leads to increase in the X-ray "d" values. D.t.a. studies indicate that Beta (Fe) has lower thermal stability, of the crystal than does the AI analog. The framework i.r. bands are shifted to lower frequencies on Fe incorporation in the lattice. E.s.r. and magnetic susceptibility data indicate the presence of well-dispersed tetrahedral Fe3+. Beta (Fe) possesses significant activity in the isomerization of m-xylene. Keywords: Zeolite beta; iron-silicates; isomorphous substitution by Fe; Fe-Beta; zeolite synthesis

INTRODUCTION

EXPERIMENTAL

Isomorphous substitution of A1 or Si in the zeolite framework by other elements such as Ga, Be, B, Fe, Cr, P, and Mg has been reviewed by Barrer. 1 The substitution of AI by Fe in the ZSM-5 framework is now well established. 2-5 Szostak and Thomas 6 were the first to deliberately synthesize a condensed-phase zeolite, sodalite, with significant quantities of iron in the framework (SiO2/Fe203 = 6-30). Recently, we reported 7 the synthesis of NaY, wherein AI 34- has been partially replaced by Fe ~+. In this paper, we report the direct synthesis of a crystalline ferrisilicate zeolite with the framework structure of zeolite beta and substantially free of aluminium. The synthesis, structure, and catalytic properties of zeolite beta are already documented in the literature, s-ll Zeolite beta is the only high-silica zeolite to have a fully threedimensional, 12-ring pore system. 9 It is also the only zeolite to have a near-random degree of stacking defects and yet maintain full sorption capacity. In addition, it is the only large pore zeolite to have chiral pore intersections 9 with potential application in the separation of chiral molecules. In view of the above, the synthesis and characterization of ferrisilicate analog of beta zeolite would be of both scientific and industrial interest. Such a synthesis has not, so far, been reported in the literature.

Synthesis of zeolites

Address reprint requests to Dr. Kumar at the National Chemical Laboratory, Pune 411 008, India. Received 27 December 1988; accepted 15 June 1989

(~) 1990 Butterworth Publishers

The synthesis of ferribeta (ferrisilicate with the crystalline structure of beta zeolite 9'~°) was carried out hydrothermally using tetraethyl orthosilicate (TEOS, 98%, Aldrich), tetraethyl a m m o n i u m hydroxide (TEAOH, 20% aqueous solution, Alfa), ferric sulfate (GR), and sodium hydroxide (AR grade). In a typical preparation, 83.2 g of TEOS were added slowly under stirring to 40 g of T E A O H in order to partially hydrolyse TEOS. The resulting mixture was slowly added to a solution of 2.5 g of ferric sulfate in 10 g of doubly distilled water under vigorous stirring. To the mixture obtained in this manner, a solution of 1.6 g of NaOH in 100 g T E A O H (20% aq. solution) was added. The slurry was stirred for 24 h at 333 K, in an open vessel. In this way, the ethanol formed during hydrolysis of TEOS was allowed to evaporate. The resulting gel was white in color, indicating the absence of brown ferric hydroxide. This gel was then transferred to a stainless-steel autoclave and kept in an oven at 393 _+ 1 K for 12 d. The autoclave was then removed from the oven and quenched in cold water. The contents were filtered, washed, and dried at 393 K in air for 8 h. The white material was then calcined at 713 K for 16 h in a flow of air to remove the organics and obtain the Na-form of the zeolite. The catalytically active protonic form was obtained by repeated ionexchange with a dilute solution of ammonium hydroxide, drying, and calcination at 713 K for 16 h in a flow of air. The aluminium analog, beta(A1), was synthesized according to procedures reported earlier, s-t0

ZEOLITES, 1990, Vol 10, February 85

Iron-silicate beta: Ft. Kumar et al.

Zeolite characterization The chemical analyses were performed by a combination of-wet chemical, atomic absorption (Hitachi Z-800), and ICP (Jobin Yuon-JY-38 VHR) methods. The crystalline-phase identification was carried out by XRD (Philips, PW-1710). The zeolites were further characterized by scanning electron microscopy (Cambridge), thermal analysis (Netsch, Model STA 490), FTi.r. (Nicolet 60 SXB), and e.s.r. (Bruker E-2000) spectroscopies. The framework i.r. vibration spectra were recorded by the Nujol technique. The solid-state MAS (for 29Si and 27A1) spectra were recorded at 295 K using a Bruker MSL-300 FTn.m.r. spectrometer. For 29Si'and 27A1, Bloch decays were averaged 2400 times before Fourier transformation to get spectra with sufficient S/N. While acquiring 29Si spectra, a recycle time of 3 s was found to be sufficient to give fully relaxed spectra. M A S was kept at 3.5 KHz. "29Si spectra were measured using TMS as the primary reference, whereas an aqueous solution of AICI3 provided the reference peak for 27A1. Adsorption measurements were carried out gravimetrically using a McBain balance in a conventional BET system. The magnetic susceptibility measurements, in the range of 94-297 K, were measured using the Faraday-balance (Cahn-Ventron, Cerritos, CA USA). The protonic form of the zeolite, prepared as mentioned above, was used for all catalytic experiments. The isomerization of m-xylene on the zeolite samples was evaluated in a downflow, tubular, silica reactor using 1 g of the powdered zeolite (10-20 mesh). The products were analyzed by gas chromatography (Shimadzu) using a 5% bentone-34 + 5% DIDP column.

Table 1 "d" values of alumino- and ferribeta zeolites Beta (AI)

Beta (Fe)

d, n m

//'/o

d, n m

///o

1.127 0.749 0.655 0.530 0.499 0.476 0.411 0.392 0.349 0.330 0.307 0.300 0.291 0.266 0.257 0.206

22 4 5 .4 18 100 9 20 4 22 8 6 3 8

1.147 0.762 0.665 0.535 0.493 0.479 0.415 0.396 0.352 0.332 0.310 0.303 0.294 0.268 0.259 0.207

23 4 5 3 17 100 8 16 5 16 4 5 3 6

seen on isomorphous substitution of AI by Fe in the faujasite lattice. 7 T h e XRD pattern of beta (AI) matched very closely those published earlier.ll It may be noted that a pure silica polymorph, with the structure of beta zeolite, has not been synthesized so far. In fact, Perez-Pariente et al. 1m2 concluded that no zeolite beta can be made when using tetraethyl orthosilicate as the source of SiO2 in an aluminiumfree system. This conclusion is also supported by our earlier studies.13 Hence, the crystalline material with the structure of beta that was obtained (in the present study) in systems free of AI but containing Fe is unlikely to be the pure silica polymorph of beta but its ferrisilicate analog.

Scanning electron microscopy RESULTS AND DISCUSSION

Chemical composition The chemical composition of the as-synthesized zeolite, in terms of the mole ratios of the oxides, could be expressed as (TEA20)0.4:(Na20)0.s:(Fe2Oa): (SiO2)a7:20(H20). Because of the highly pure source of silica used (tetraethyl orthosilicate), the concentrati.on of AI in the zeolite was negligible (SiO2/Al2Oa > 2000). AI could not be detected by n.m.r, either in the sample of ferribeta. The color of the as-synthesized sample as well as that calcined at 753 K in dry air was completely white, indicating the absence of the colored oxides of iron, at least in bulk form outside the zeolite crystals. The samples developed a buff, light brown color only on prolonged hydrothermal treatment.

X-ray diffraction The X-ray diffraction patterns of the ferribeta sample along with that of a conventional beta (AI), [SiO2/Al2Oz = 40] were found to be similar. The corresponding "d" values are shown in Table I. Isomorphous substitution of Fe for A1 increases the "d" values, indicating lattice expansion due to the larger Fe atoms. A similar expansion of the lattice was also

86

ZEOLITES, 1990, Vol 10, February

The crystals of ferribeta (about 1 I~m in length) are ellipsoid in shape. The SEM photographs also indicated the absence of amorphous matter external to the zeolite pore system. The beta (A1) samples were about 0.3-0.5 l~m cubes) 4

Thermal analysis The relative thermal behavior of as-synthesized ferribeta vis-a-vis beta (Al)'was evaluated by d.t.a./t.g. (Figure 1). A detailed analysis of the thermal behavior of beta (AI) has been published earlier. 12'x4 On substitution of AI by Fe, the exothermic weight changes due to the loss of occluded T E A O H and TEA + cations are shifted to lower temperatures (Figure 1). The total weight loss was, however, similar (around 18 wt%) for both the samples. The broad exothermic peak at 873 K, assigned by Perez-Pariente et al. ~2 to the oxidative decomposition of organic materials occluded in the zeolite pores, shifts to 773-823 K in ferribeta. The presence of TEA + species as charge-balancing cations indicates the presence of negatively charged (FeO4) tetrahedra in the zeolite framework.

Framework i.r. spectroscopy The framework i.r. spectra (in the region 200-1300

Iron-silicate beta: R. Kumar et al. B

A

I t I

f~J I I I

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x

B

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! / I

//

I 1200

I000

1 800

600

400

cfff ~

-.-~1

i 373

i 373

i 773 TEMP

i 975

i 1173

K

Figure 1 D.t.a. (bottom) and d.t.g. (top) of beta (AI) and ferribeta (A and B, respectively)

cm -1) of beta (AI) and ferribeta are shown in Figure 2). The values of some major absorption bands are compiled in Table 2. On introduction of Fe in the zeolite, most of the bands are shifted to lower wavenumbers in agreement with expectations. For example, the bands at 1175 and 1075 cm -x, due to asymmetric stretching vibrations of S i - O - T (T = A1) are shifted to 1168 and 1060 cm -l on isomorphous substitution of AI by the heavier Fe atom in the lattice framework. E.s.r. s p e c t r o s c o p y Although an e.s.r, signal around g -- 4.3-4.4 cannot be used to confirm the presence of Fe a+ in zeolite lattice positions, t5 the observation of such a signal is a necessary consequence of such a presence and may, hence, lend additional support to any postulate of isomorphous substitution of Al by Fe. The e.s.r. spectra of the ferribeta samples (Figure 3) reveal two main signals at g = 2.0 and 4.4, respectively. The latter is assigned to tetrahedral Fe 3+ possibly in lattice positions. The enhanced intensity of this signal at lower temperatures as well as its relative insensitivity to oxidation-reduction treatments suggests that it arises from Fe 3+ ions in tetrahedral lattice positions rather than nonframework positions.

Magnetic susceptibility The magnetic moments of the as-synthesized ferribeta were measured by the Faraday balance method between 94 and 297 K. T h e values (in Bohr magnetons) were 5.8 and 5.5 at 04 and 297 K, respectively.

Figure 2 I.r. framework vibration spectra of beta (AI) and ferribeta (A and B, respectively)

There was no significant difference between the values for the as-synthesized samples and those calcined in dry air below 773 K. Non-interacting Fe a+ ions in a diamagnetic matrix are expected to have a magnetic moment below about 6 B.M. The observed values of 5.5-5.8 B.M. for ferribeta and the relative insensitivity of these values to temperature changes indicate the absence Of a significant concentration of iron-oxide phases. The latter, if present, would exhibit much higher values of the magnetic moment (depending on the degree of agglomeration of the iron-oxide phase). In addition, the value of the magnetic moment would also increase markedly at lower temperature. Hence, the magnetic susceptibility data indicate that (1) iron-oxide phases, like Fe2Oa or FeaO4, are absent in our sample of ferribeta and (2) that the Fe a+ ions are present in a magnetically dilute environment. Both these observations are consistent with, but do not necessarily confirm, the isomorphous substitution of Fe a+ in the beta lattice. They are, however, req.uisite corollaries to any postulate of such substitution. Table 2 Framework i.r. vibrations (cm -1) Beta (AI)

Beta (Fe)

1215 (w) 1175 (w) 1075 (s) 790 (s) 740 (w) 615 (sh) 580 (s)

1205 (w) 1168 (w) 1060 (s) 777 (s) 730 (w) 605 (sh) 575 (s) 530 (w) 500(m) 445 (s) 430 (s)

520 (m) 450 (s) 430 (s) w = weak; s = strong; m = medium; sh = shoulder

ZEOLITES, 1990, Vol 10, February 87

Iron-silicate beta: R. Kumar et

/

al.

,

"

\

I

\\

i

ix t 0=2.O i

/'

/

/

/

/

/

/

/

i

f

/

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Figure 3

E.s.r. spectra of ferribeta at 297 and 94 K (A and B, respectively)

Solid-state M A S n.m.r, spectroscopy The 298i and 27A1MAS n.m.r, of beta(A1) are shown in Figure 4A. The spectra of ferribeta (Figure 4B) shows the absence of aluminium in the sample. T h e broadening of the 29Si signal in ferribeta compared to that in beta(A1) arises from the Si-O--Fe nuclearelectron coupling in the former zeolite.

Adsorption studies T h e a d s o r p t i o n o f n- a n d c y c l o h e x a n e , nbutylamine, and H20 in the sodium forms of beta(A1) and ferribeta are shown in Table 3. They have similar adsorption capacities. Benslama et al.18 had reported

Si (OAt)

A

values for adsorption of normal and cyclohexane on beta (A1) of 18.7 and 20.0, wt%, respectively. The original patent of Wadlinger et al. s reports values of 16.6 and 19.4 wt% for adsorption of normal and cyclohexane, respectively, for beta (AI). C a t a l y t i c studies The isomerization of m-xylene over Fe-beta and Al-beta, is shown in Table 4. At lower conversion levels, the concentration of 1.3.5 trimethylbenzene (TMB) is around 20% among the three TMB isomers lower than the equilibrium value. 16 Further, the selectivity for isomerization [(p-xyl. + o-xyl.)/m-xyl. converted], controlled by restricted transition-state shape selectivity, exhibited by Fe-beta is comparable to that exhibited by Al-beta (Table 4). This manifestation of shape selectivity is evidence that the reaction occurs on active sites located within zeolitic pores. This reaction is known to be catalysed by Br6nsted acid sites) 6 Since these sites could have been generated only by protons associated with Fe 3+ in lattice positions, the occurrence of this reaction over the sample of ferribeta is strong evidence of isomorphous substitution of Fe ~+ for A13+ in the beta lattice. CONCLUSIONS T h e experimental r esuhs may be summarized as follows: A crystalline ferrisilicate (SiO2/Fe2Os = 37),

100

PPm (TMS)

I00

0

0

PPrn [AI(H20) 6 ]34

Table 3 Adsorption (wt%) on alumino- and ferribeta zeolites (Temp. [K] = 298; P/Po = 0.5) Adsorbate

Rgure 4 MAS n.m.r, of beta (AI) and ferribeta (A and B, respectively) *Spinning side bands

88 ZEOLITES, 1990, Vol 10, February

n-hexane Cyclohexane n-butylamine H20

Na-beta (AI)

Na-beta (Fe)

19.2 22.5 21.0 27.9

18.0 18.7 18.3 23.6

Iron-silicate beta: R. Kumar et al. Table 4 Isomerization of m-xylene over H/Fe-beta and H/AI-beta Zeolite Temp. (K) Cony. (%) Products~ wt% Toluene Ethylbenzene p-Xylene m-Xylene o-Xylene 1.3.5 TMB a 1.2.4 TMB 1.2.3 TMB 1.3.5 TMB/TMB Sel. isom. u

H/Fe-Beta (Si02/Fe20z = 37) 623 5.1

643 14.5

0.4 0.2 1.8 94.9 2.2 0.1 0.4 0.20 0.78

1.3 0.1 5.1 85.5 5.6 0.5 1.7 0.23 0.74

663 18.2 2.0 0.2 6.4 81.8 7.0 0.6 1.8 0.2 0.23 0.73

683 24.0 2.9 0.2 8.0 76.0 9.2 0.8 2.6 0.2 0.22 0.71

H/AI-Beta (Si0z/AI203 = 40) 703 30.3 3.8 0.1 9.7 69.7 10.8 1.3 4.1 0.5 0.22 0.67

473 5.3 0.6 . 2.0 94.7 1.9 0.2 0.6 0.25 0.74

488 14.7

518 23.8

2.0 .

3.5 .

5.4 85.3 5.1 0.6 1.5 0.1 0.27 0.71

. 8.1 76.2 8.0 1.1 2.9 0.2 0.26 0.67

548 33.4

558 38.6

6.5

10.9

10.3 66.6 9.9 1.8 4.4 0.5 0.27 0.60

10.2 61.4 9.9 2.2 4.7 0.6 0.29 0.52

.

Feed: m-xylene + Hz (1:4, mole), WHSV --- 3.5 h -1, pressure = atmospheric a TMB = trimethylbenzene b Sel. isom. = (p-xyl. + o-xyl.)lm-xyl, converted

with the lattice framework of zeolite beta, has been synthesized using tetraethyl orthosilicate, free from A1, as the source of SiO2. XRD data indicate a slight lattice expansion, suggesting incorporation of Fe a÷ in lattice positions. The shift to lower frequency of framework vibrations is also consistent with this suggestion. Scanning electron microscopy and adsorption results support the absence of amorphous matter both outside and inside the zeolitic pore system. The color of the sample, both in the as-synthesized and calcined forms, was white in color, confirming the absence in it of oxides of iron in significant quantities. A signal at g = 4.4 in the e.s.r, spectra of the sample confirms the presence of Fe 3+ in tetrahedral coordination with oxide ions; whereas magnetic susceptibility data suggest that these Fe a+ ions are well isolated from each other and that F e - O - F e interactions are absent in the sample. Solid-state MAS n.m.r. spectra of 295i reveal the signal-broadening that would be expected due to S i - O - F e nuclear-electron coupling. Finally, the ferribeta sample exhibits significant catalytic activity and shape selectivity in a typical Br6nsted acid-catalysed reaction like the isomerization of m-xylene. On the basis of all the above evidence, it is concluded that isomorphous substitution of Fe a+ for Al ~+ in the crystalline lattice of zeolite beta has been demonstrated.

REFERENCES 1 Barrer, R.M., H~/drothermal Chemistry of Zeolites, Academic Press, London, 1982, p. 251 2 Kovenhoven, H.W. and Stork, W.H.J., US Pat. 4,208,305, 1982 3 Ratnasamy, P., Borade, R.B., Sivasankar, S., Shiralkar, V.P. and Hegde, S.G. in Proceedings of the International Symposium on Zeolite Catalysis, Si6fok, Hungary, 1985, p. 137 4 Ball, W.J., Dwyer, J., Garforth, A.A. and Smith, W.J. Stud. Surface ScL Catal. 1986, 28, 137 5 Szostak, R. and Thomas, T.L.J. Catal. 1986, 100, 555 6 Szostak, R. and Thomas, T.L.J. Chem. Soc,, Chem. Commun. 1986, 113 7 Ratnasamy, P. Kotasthane, A.N., Shiralkar, V.P., Thangaraj, A. and Ganapathy, S. Paper presented at the ACS Symposium on Advances in Zeolite Synthesis, Los Angeles, CA, USA, September 25-27, 1988 8 Wadlinger, R.L., Kerr, G.T. and Rosinski, E.J. US Pat. 3 308 069 (1967) 9 Tracy, M.M.J. and Newsam, J. M. Nature 1968, 352, 249 10 Higgins, J.B., La Pierre, R.B., Schlenker, J.L., Rohrman, A.C., Wood, J.D., Kerr, G.T. and Rohrbaugh, W.J. Zeolites 1988, 8, 446 11 Perez-Pariente, J., Martens, J.A. and Jacobs, P.A. Zeolites 1988, 8, 46 12 Perez-Pariente, J., Martens, J.A. and Jacobs, P.A. AppL Catal. 1987, 31, 35 13 Bhat, R.N. and Kumar, R. J. Chem. Biotech., in press 14 Hegde, S.G., Kumar, R., Bhat, R.N. and Ratnasamy, P., Zeolites, 1989, 9, 231 15 Kucherov, A.V. and Slinkin, A.A. Zeolites 1988, 8, 10 16 Benslama, R., Fraissard, J. Albizane, A., Fajula, F. and Figueras, F. Zeofites 1988, 8, 196 17 Csicsery, S.M. PureAppL Chem. 1986, 58, 841

ACKNOWLEDGEMENT The work was partly funded by UNDP.

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