Synthesis and Characterisation of Ferrisilicate Zeolites

43

Synthesis and Characterisation of Ferrisilicate Zeolites

R. Kumar and P. Ratnasamy National Chemical Laboratory, Pune 411 008, India

ABSTRACT Ferrisilicate zeolites wherein iron ions replace silicon in the lattice framework have potential as catalyst in various conversion processes. During the past decade ferrisilicate analogs of sodalite, MFI, MEL, MTT, EUO, M'IW, FAU, BETA, MOR and LTL have been synthesised and characterised by various physicochemical techniques as well as catalytic reactions. After a review of the general synthesis procedures a list of criteria is presented to confirm the location of Fe in the zeolite framework. Examples are provided to illustrate the utility of the various characterisation techniques. INTRODUCTION The first isomorphous replacement in the zeolite framework was reported by Goldsmith in 1952 in the synthesis of a germanium containing thomsonite wherein Ge replaced Si in the lattice [l]. Later, Barrer et al. [2] reported a number of Ga- and Ge- bearing zeolites. In the past decade the isomorphous 3+ substitution of many tri-, tetra- and pentavalent cations (B%, Fe%, Ga , Ge4+, Ti4+ and P5+ ) in various zeolite frameworks has been reported. How does the isomorphous substitution of A13+ or Si3+ in the zeolite lattice by other ions affect their structural stability, acidity and catalytic performance ? The present paper deals with ferrisilicate analogs of various medium (10-ring) and large (12-ring) pore zeolites. The ionic radii of Si4+, A1% and Fe% are 0.039, 0.057 and 0.067 nm, respectively. In addition, Fe3+ can also undergo a change in its oxidation state thereby leading to a lowering of the stability of the crystal structure. The isomorphous substitution of Si by Fe in the lattice structure of ZSM-5 [3-61, ZSM-23 [7], sodalite [8], beta [9] and FAU [lo] has been reported. More recently, we have synthesised and characterised the ferrisilicate analogs of ZSM-11, EU-1, ZSM-12, L and mordenite. We describe herein the general procedures for synthesising and characterising the ferrisilicates.

44 R. Kurnar and P. Ratnasarny

EXPERIMENTAL Synthesis During the synthesis of ferrisilicates in aqueous systems, the following equilibria prevail : [Fe(H20>6I3+

+

Si(OH>4

[Fe(H~0)6]~~t H20

[FeSiO(OH)3Izt

Hyd t H30t

(1)

+ H30+

(2)

[Fe(OH>(H20>512t

The objective of the synthesis would be to maximise the first reaction (leading to ferrisilicates) and suppress the formation of hydroxides of iron by the latter reaction by operating at low pH, using aluminium free source of Si and adjusting the reaction conditions to maximise the concentration of monomeric/short chain silicate species. Usually the monomeric/short chain silicate species is added to an acidic solution containing the Fe3+ ions to form the ferrisilicate complex.

The organic

base, as a template, is added after the formation of the ferrisilicate gel. After adjusting pH to the desired value, the amorphous gel is converted into the crystalline zeolite by crystallisation in an autoclave at elevated temperatures. Ferric nitrate is the usual source of Fe. Tetraethylorthosilicate (TEOS) is a preferred source of Si even though sodium silicate, silica gel and silica sol can also be used. By way of illustration the synthesis of the ferrisilicate analog of ZSM-11 is given below : 40 g of

TEOS was added slowly to a solution containing 2.0

g Fe(N03)3.9H20,

30 g doubly distilled water and 6 . 2 g H2S04 under stirring. To the above mixture a solution of 7.6 g 1,8 diaminooctane in 40 g water was added. Finally, 4.3 g NaOH dissolved in 25 g water were added under vigorous stirring. The resulting white gel was stirred at 298 K for 1 h before transferring it into a stainless steel autoclave (200 ml capacity). The crystallisation was carried out statically at 433 K for 8 days. The as-synthesised zeolite was carefully calcined at 753 K (heating rate ZO/min) first in dry nitrogen for 8 h and then in air for an additional period of 8 h. The protonic form of the zeolite was obtained by repeated ion exchange with 1N aqueous solution of NH4C1 (tNH40H) (pH=7-8) at 343 K for 2 h, drying and calcining.

Characterisation The chemical analyses were done by a combination of wet chemical, atomic absorption (Hitachi 2-800) and ICP (JY-38 VHR) methods. The crystalline phase identification was carried out by XRD (Philips PW-1710 Cu Ka). The zeolites were further characterised by scanning electron microscopy (Cambridge, Stereoscan 400), thermal analysis (Netsch, Model STA 490), ESR

Synthesis and Characterisation of Ferrisilicates 45

(Bruker E-2000), MASNMR (Bruker MSL-300), FTIR (Nicolet 60SXB) and Mijssbauer spectroscopies, magnetic susceptibility (Cahn Ventron), adsorption (McBain balance) and catalytic measurements. The procedures have been fully described in our earlier publications [3,6,7,9,10,12-161. RESULTS AND DISCUSSION Synthesis and characterisation Ferrisilicate analogs of zeolites are of potential utility as monofunctional (acidic) or bifunctional catalysts. Analogous to A1%, the replacement of Si4+ by FeZI. generates Bronsted acidity [7,14]. The use of ferrisilicate zeolites as bifunctional catalysts is due to the fact that under severe steaming conditions part of the Fe can be removed from the lattice framework and dispersed as finely divided iron oxide particles within the zeolitic pore system [17]. In this case, the material can function both as an acidic catalyst (due to that part of Fe3+ still in the lattice framework) and also as a redox catalyst due to the presence of finely dispersed Fe203/Fe304 in its pore system [17]. Table i summarises the list of ferrisilicate zeolites that have been prepared to-date by direct synthesis in basic media. Ferrisilicate pentasil zeolites have also been synthesised hydrothermally in an acidic, fluoride-containing medium [ 18,191. Such samples, however, sometimes, exhibit lower Bransted acidity (than those prepared in basic media). This may be due to the simultaneous replacement of 02- by F- thereby eliminating the need for charge-balancing cations like Na+ or H+ [19]. When ferrisilicate analogs of known zeolites are made it is essential to establish the presence of Fe3+ in the lattice framework. Towards this objective we have used a variety of techniques each one of which gives specific structural/textural information about the sample's characteristics (Table 2). Table 3 lists physical properties of some of the synthetic ferrisilicates. Among the known ferrisilicate zeolites, (Fe)-ZSM-5 is the most documented [ 3-6,18,19,21-271. The material has been synthesised from both basic [ 31 and fluoride-containing media [ 191. Incorporation of Fe in lattice positions has been established by XRD [4], framework IR [4], Massbauer [25], ESR [ 3,271, and uv-vis [ 191 spectroscopies, ESCA [ 31, ion exchange [ 251, magnetic susceptibility [ 261 and catalytic activity [9,15] measurements. We have recently synthesised the ferrisilicate analog of pure ZSM-11 (free from ZSM-5) [ll]. The location of Fe3+ ions in the MEL lattice has been confirmed by all the above techniques. For example, the increase in the unit cell parameters of the MEL lattice on Fe incorporation is shown in Fig. 1.

46 R. Kumar and P. Ratnasamy

Table 1. Synthesis conditions for ferrisilicate analogs of zeolites. Zeolite

Source of Si

MFI MEL

NagSiO3

nos

MTT

EUO FAUa MORa

Silica NagSiOg -do-

9

TEOS

MTW

Na2SiOg TEOS Silica

BETAa

LTLb

Organic template Triethylbutyl amm. bromide TBA-OH/1, 8-diaminooctane Pyrrolidine Hexamethonium dibromide (Seeds) TEA-Br Methyltriethyl amm. bromide TEA-OH

Temp.

(K)

453 443

453 443 373

443 443 393 443

Time (d)

Ref.

3 9 2

6 11 7

1 4

10 13 11

4

4

12 5

12

9 11

aFe source was ferric sulphate; for other zeolites ferric nitrate was used. bThe Si/A1 in L and Y zeolites varied in the range 4-5 and 2.3-3 respectively. The other ferrisilicates were substantially free from Al. Table 2 .

Techniques for characterising ferrisilicate zeolites.

Techniques

Relevant information

1 Color 2 Chemical analysis 3 XRD

4 Electron microscopy/ EDAX 5 Adsorption

6 Thermal analysis

7 IR spectra

:

Hydroxyl bands Framework

8 ESR

9 Magnetic moment 10 Mdssbauer spectra 11 ESCA 12 Ion-exchange capacity 13 Acidity 14 Phosphorescence 15 UV-VIS spectra 16 29% MASNMR 17 Catalytic activity in acid catalysed reactions

White color indicates the absence of bulk hydroxides/oxides of iron Fe and A1 content Crystallinity/phase purity; lattice expansion due to Fe incorporation Absence of amorphous matter outside the crystalline phase; distribution of Fe. Absence of amorphous matter within the pores of the zeolite Temperature of crystal collapse; pattern of evolution of organics Shift to higher wave-numbers of bridged hydroxyls (Fe-OH-Si) Shift to lower wave-number of sym. and asym. stretching frequencies (Fe-0-Si instead of Si-0-Si) Peak at g = 4.3 due to distorted Td Fe% Insensitivity of peak to reduction conditions Bohr magneton 5.6-6.0 IS = 0.3-0.4 mm/s at 4.2 K and IS = 0.2-0.3 mm/s at 298 K Absence of extra Ols peak due to Fe-oxides Quantitative criterion in TPD of ammonia [lS] Lower,,T 5000 A : F e k in Td;7000 A : Fek in [20] IODq = 7500-8500 cm-l(see ref.19, p.91) Shortening of spin-spin relaxation time [ 103 Quite useful, provided the ferrisilicates are A1 free

Synthesis and Characterisation of Ferrisilicates 47

Table 3.

P h y s i c a l p r o p e r t i e s of f e r r i s i l i c a t e analogs of z e o l i t e s

Property

MFI

MEL

MTT

ELI0

MTW

MOR

BETA

FAU

LTL

Si/Fe ESR, g value Mag. moment p BM, RT 97 K Mbssbauer IS m/s, RT 4.2 K Ion-exchange K+/Fe02, % Adsorptiona W t . %, water n-hexane Cyclohexane

36 4.3

35 4.3

58 4.4

18 4.4

65 4.3

09 4.3

17 4.4

17 4.3

10 4.4

5.8 5.6

5.8 5.6

5.9 5.5

5.7 5.5

5.8 5.7

5.8 5.6

0.24 0.34

0.22 0.32

28.0

18.9

0.25 0.35

0.26 0.33

79

82

86

76

9.8 11.0 5.1

8.5 11.3 5.2

5.8 8.5 3.5

9.4 11.0 5.7

75 7.0 7.5 6.0

7.2 4.1 4.8

23.6 18.0 18.7

.

aP/Po = 0.5 (except i n t h e case of LTL where PIPo was 0.8) T = 298 K

A 5

10

DELAY

5

(TI m

10

Sec.

Fig. 2. Spin-spin decay of 29Si s i g n a l s of (A):(Al)-FAU and (B): (Fe)-(A1)-FAU z e o l i t e s . Lines 1-4 r e f e r t o S i (3Al)-Si (OAl) signals respectively.

Fe 1 (Fe t Si 1 Fig. 1.

Unit c e l l parameters as a f u n c t i o n of Fe-content of (Fe)-ZSM-ll zeolites.

48 R. Kumar and P. Ratnasamy

(Fe)-ZSM-23 free from aluminium has been synthesised [7] and the presence of Fe in the lattice framework confirmed by spectroscopic (XRD, IR, ESR and XPS), DTA/TG, magnetic susceptibility, ion exchange and catalytic activity measurements. (Fe)-beta has also been similarly prepared and characterised [9]. Isomorphous substitution of Fe in the faujasite lattice has been demonstrated [lo] using various techniques including solid state MASNMR. NMR spin-echo experiments indicated that the spin-spin relaxation time of 29Si is shortened due to the presence of Fe in the FAU lattice framework

[lo].

In

this

experiment,

the

180'

pulse

refocuses

the

inhomogeneity effects contributing to the line broadening. Thereafter, any contribution to Si line-width due to susceptibility effects (arising from occluded Fe203 or Fe304, for example) will be refocused and the decay of the 29Si spin echoes will be determined only by spin-spin relaxation of Si nuclei. Fig. 2 presents the decay of the spin echo Si signal intensity for (A1)-FAU (Fig. 2A) and (Fe)-(A1)-FAU, the latter containing Fe and A1 ions in the framework (Fig. 2B). The T2 (spin-spin relaxation time) for the sample containing Fe was only 2.2 compared to 7.8 msec. for the Al-analog. This lower value comes from the Si-Fe nuclear-electron coupling and provides conclusive evidence that Fe is in the framework. It may be mentioned here that when Fe3+ ions are introduced by ion exchange (for Nat ), the resulting samples do not exhibit the shortening of T2 values. This is because Si-0-Fe "through bond interactions" are present when Fe is in the lattice and are absent when Fe is present only as a counter-ion outside the lattice framework. In all these zeolites, DTA studies revealed that the ferrisilicates have lower thermal stability compared to their Al-analogs.

Al-free ferrisilicate analogs of mordenite have recently been

synthesised using tetraethyl ammonium bromide [13] and the presence of Fe in lattice positions demonstrated by various techniques. Senderov et al. [26] have also reported the synthesis of mordenite containing both A1 and Fe. The synthesis, characterisation and catalytic properties of (Fe)-EU1 have also been described [12,29]. Fig. 3 shows a plot of reciprocal gram susceptibility against temperature for (Fe)-EU-1. The data could be fitted to the Curie-k'eiss Law and the Weiss temperature was close to 0 K indicating the absence of significant interaction between Fe9 ions. These results confirm the high dispersion of FeZC ions (probably in lattice positions) in these materials. 57Fe Mossbauer measurements on the as-synthesised samples of (Fe)-EU1 and (Fe)-beta zeolites at 298 K and 4.2 K and at 4.2 K with externally applied magnetic field (4.13T) are presented in Fig. 4 (curves A,B and C,

Synthesis and Characterisation of Ferrisilicates

49

(Fe)- EU- I 75

A

! AS -SYNTHESIZED

0

n

-0 x

CALCINED

50 25 -

0II

-50

I

0

I

I

100

50

150

I

200

I 300

I

250

TEMP, K

FIG.3 ! R ECIPROCAL

GRAM

SUSCEPTI BlLlTY OF ( F e )

- E U- I

z

-

0 v)

G z

v)

z a

K

I-

w

> I-

a

-I W

0:

- 10

0

10 -10

0

m

ISOMER S H I F T ( 6 1 , m m I S

FIG.4-MOSSBAUER

SPECTRA OF ( F e ) EU-I AND (FePBETA ZEOLITES

50 R. Kumar and P. Ratnasamy

respectively). The values of the isomer shifts (0.26 and 0.22 at 298 K and 0.33 and 0.32 at 4.2 K, for (Fe)-EU-1 and (Fe)-beta, respectively) and quadrupole splitting (0.00 mm/sec) are indicative of tetrahedrally coordinated Fek species [ 301 having insignificant distortion in the local tetrahedral surroundings of ferric ions. The Mossbauer spectrum at 4.2 K in the presence of externally applied magnetic field (perpendicular to they-rays) (Fig. 4, curve C) shows the characteristic paramagnetic hyperfine structure arising due to the 3-crystal field split states +5/2> +3/2)+1/2) of 6S5/2Fe3t ions. The average value of the internal magnetic field (Hint = 46.8 T) lies well within the range specified for tetrahedrally coordinated ferric ions [31]. Catalytic properties Changes in shape selectivity due to the isomorphous substitution of A1 by the larger Fe has not, so far, been unequivocally been established. However, differences in catalytic activity, selectivity and stability between alumino- and ferrisilicate zeolites arising from the presence of weaker acid sites in the latter [14] have been noted [3,7,19,21,22]. In the conversion of methanol to olefins [21], for example (Fe)-ZSM-5 yields more C2-C4 olefins than the Al-analog (Table 4). In the hydrodewaxing of gas oil, (Fe)-ZSM-5 has a lower activity than the Al-analog (as seen from the higher temperature (641 vs 623 K) required to dewax the oil to the same pour point level [21]). However, the larger C5+ yield observed over the less acidic ferrisilicate was probably due to the lower secondary cracking over it. The lower acid strength of the ferrizeolites has implications also in the relative rates of deactivation (vis-8-vis the Al-analogs) in those reactions where bulky polyalkylaromatics formed within the pore system can Table 4. Conversion of methanol to olefins over H-(Al)-ZSM-5 and H-(Fe)-ZSM-5. Feed : 80 % (v/v) methanol in water; Temp. : 723 K; WHSV : 2.2 h-l Press. : atm; Methanol conversion : 100 %; Dimethyl ether : 0.0 %.

SiO2/M2O3 Average crystal size, p Hydrocarbons, w t . % Ethylene Propylene Butenes C1-C4 alkanes

c5+

H-(Al)-ZSM-5

H-(Fe)-ZSM-5

86 2-3

72 2-3

3.1 4.6 1.o 45.7 45.6

10.3 21.6 15.5 15.5 37.0

Synthesis and Characterisation of Ferrisilicates 51

lead to deactivation of the catalyst. Fig. 5 illustrates the relative deactivation rates of (A1)- and (Fe)-ZSM-ll in the disproportionation of ethylbenzene to benzene and diethylbenzenes. Even though the initial activity of the Al-analog was higher the catalyst deactivated faster. It may be mentioned here that the distribution of ,the three (para, meta and ortho) diethylbenzene isomers was similar on both the catalysts. Hence, the shape selectivity of both the Al- and Fe- zeolites was similar. The observed differences in their deactivation characteristics is probably due to differences in the strength of their acid sites. Ferrizeolites can exhibit bifunctional catalytic behaviour when part of the Fe3+ ions are removed from framework positions (by hydrothermal treatment, for example). In such samples, finely dispersed iron oxide particles coexist with Fe3+ ions in lattice positions. While the latter can take part in acid-catalysed reactions (like the disproportionation of ethylbenzene to benzene and diethylbenzenes), the former can give rise to redox activity, for example, in the dehydrogenation of ethylbenzene to styrene.

t

0

40.

: 648

3*5 7.0

0 ; 648

i

WFe) -ZSM-II

-

0

0

I

I

a

1

I

I

4 6 2 TIME ON S T R E A M ( T O S ) , h

-

I

Fig. 5. Ethylbenzene conversion against time-on-stream : -0: H-(Fe)-ZSM-11, and *:H-(Al)-ZSM-ll

I

8

I

A0

52 R. Kumar and P. Ratnasamy

ACKNOWLEDGEMENT This work was partly funded by UNDP. REFERENCES 1 J.R. Goldsmith, Min. Mag., 29 (1952) 952. 2 R.M. Barrer, J.W. Baynham, F.W. Bultitride and W.M. Meier, J. Chem. SOC., (1959) 195. 3 P. Ratnasamy, R.B. Borade, S. Sivasanker, V.P. Shiralkar and S.G. Hegde, Acta. Phys. Chem., 31 (1985) 137. 4 R. Szostak and T.L. Thomas, J. Catal., 100 (1986) 555. 5 G. Calis, P. Frenken, E. deBoer, A. Swolfs and M.A. Hefni, Zeolites., 7 (1987) 319. 6 R.B. Borade, Zeolites., 7 (1987) 398. 7 R. Kumar and P. Ratnasamy, J. Catal., 121 (1990) 89. 8 R. Szostak and T.L. Thomas, J. Chem. SOC. Chem. Commun., (1986) 113. 9 R. Kumar, A. Thangaraj, R.N. Bhat and P. Ratnasamy, Zeolites., 10 (1990) 85. 10 P. Ratnasamy, Pk.ii. Kotasthane, V.P. Shiralkar, A. Thangaraj and S. Ganapathy, in M.L.Occelli and H.E. Robson (Eds), Zeolite Synthesis (ACS Monograph 398), Am. Chem. SOC., Washington DC, (1989) p.405. 11 R. Kumar and P. Ratnasamy, (unpublished results). 12 R. Kumar, A. Thangaraj, R.N. Bhat, M.J. Eapen, S.K. Date, E. Bill and A. Trautwein, J. Catal., (submitted). 13 A.J. Chandwadkar, R.N. Bhat and P. Ratnasamy, Zeolites (in press). 14 L.M. Kustov, V.B. Kazansky and P. Ratnasamy, Zeolites, 7 (1987) 79. 15 A.N. Kotasthane, V.P. Shiralkar, S.G. Hegde and S.B. Kulkarni, Zeolites, 6 (1986) 253. 16 R. Kumar, S.K. Date, E. Bill and A. Trautwein, Zeolites (in press). 17 V. Nair, Ph.D thesis, Georgia Inst. Tech., (1987). 18 J. Patarin, J.L. Guth, H. Kessler, G. Condurier and F. Raatz, French Patent 17711 (1986). 19 D. Lin, Ph.D thesis No.126-89 (1989), University of Claude Bernard, Lyon, France. 20 B.D. McNicol and G.D. Pott, J. Catal., 25 (1972) 223. 21 P. Ratnasamy, React. Kinet. Catal. Lett., 35 (1-Z), (1987) 219. 22 S. Sivasanker, K.M. Reddy, K.J. Waghmare, S.R. Harisangam and P. Ratnasamy, in "Proc. XI Symp. Iberoamer. Catal., Mexico (1988) 741. 23 B. Wichterlova, S. Beran, S. Bedanarova, K. Nedomova, L. Dudiwova and P. Jiru, Stud. Surf. Sci. Catal., 37 (1987) 199. 24 G. Dopplern, R. Lehnert, L. Marosi and A.X. Trautwein, Stud. Surf. Sci. Catal., 37 (1987) 215. 25 R. Szostak, Molecular Sieves, Principles of Synthesis and Identification, Reinhold (1989) 230-238. 26 A. Meagher, V. Nair and R. Szostak, Zeolites, 8 (1988) 3. 27 G.P. Handreck and T.D. Smith, J. Chem. SOC. Faraday Trans. I., 85 (1989) 319. 28 E.E. Senderov, A.M. Bychkov, I.M. Miskhin, A.L. Klyachko and H.K. Ekyer, Stud. Surf. Sci. Catal., 49 (1989) 355. 29 I.S. Dring, D.H. Hall, R.J. Oldman, J.L. Casci, W.N.E. Meredith and R.P. Tooze, Physica B, 158 (1989) 167. 30 R.L. Garten, W.N. Delgass and M. Boudart, J. Catal., 18 (1970) 90. 31 V.G. Bhide and S.K. Date, Phys. Rev., 172 (1968) 345.