The synthesis of iron cancrinite using tetrahedral iron species

The synthesis of iron cancrinite using tetrahedral iron species Kay Latham, Craig D. Williams, and Catherine V.A. Duke

School of Applied Sciences, University of Wolverhampton, Wolverhampton lWl lSD, UK Sodium ferrate (VI). as a primary source of iron, has been investigated in the hydrothermal synthesis of the zeolite cancrinite . Crystalline material w ith Fe(II1) incorporated into lattice sites was isolated from a starting gel containing 0.5 mole fraction Fe20:JAI203 • The use of X-ray powder diffraction, infrared spectroscopy, X-ray fluorescence chemical analysis, electron microscopy, thermal analysis, and magnetic measurements were all used to characterize the final product. An increase in unit cell dimensions was noted for the iron-substituted cancrinite when compared with a pure aluminum cancrinite. ft) Elsevier Science Inc. 1996 Keywords: Isomorphous substitution; iron; cancrinite; ferrate

INTRODUCTION The alkali and alkaline earth-metal ferrates are oxoanions, which contain iron in the oxidation state (VI) .1 The ferrate(VI) ion is moderately stable in strong aqueous alkali, but it decomposes rapidly in neutral or acid solution, liberating oxygen: 4[FeOSl - + IOH20 ~ 4Fe3 + + 20[OHr + 302 Sodium, potassium, rub idium, caesium, and barium salts may all be prepared in a number of ways, e.g., anodic oxidation of iron in concentrated alkali, but pure materials are only obtained by oxidation of a suspension of iron (III) oxide (hydrous) in a concentrated. alkaline hypochlorite solution: 2Fe(OH)3 + 3CIO· + 40W ~ 2FeO:- + 3Cr + 5H 20 The stability of the alkali and alkaline earth metal salts increases as the size of the metal ion increases. A major factor contributing to the stability of barium ferrate (VI) is its insolubility. Strontium and calcium ferrates are moderately soluble in water, whereas the alkali metal salts are all readily soluble. Transition metal salts are all relati vely unstable due to their tendency towards oxidation by the ferrate(VI) ion." Due to the reactivity and instability of the ferrate(VI) ion it has not been studied extensively. There are several reports of its use as an intercalation compound for the impregnation of zeolite molecular sieves, but nothing offerrate(VI) as a primary synthetic reagent. Barrer and Coie M conducted a study of the structure-directing effects of various anions upon an alkaline suspension of kaolinite. It was found that sodium ferrate(VI) resulted in the formation of a cancrinite-type material with Na" and FeO~- encapsulated within the cage system. HowAddress reprint requests to Dr. Williams. Received 13 May 1996; accepted 12 July 1996 Zeolites 17:513-516, 1996 lC> Elsevier Science Inc. 1996 655 Avenue of the Americas, New York. NY 10010

ever, a minor byproduct, designated species "H," was also isolated. Initial examination by X.ray analysis showed that this material was crystalline and suggested that a tectosilicate with some replacement of structural aluminum by iron may have been formed, but no conclusive proof was obtained. It was decided to attempt the synthesis of sodium ferrate(VI) and to examine the use of the ferrate ion in zeolite synthesis. The moderate stability offerrate(VI) in a strong aqueous base coupled with its existence as discrete , tetrahedral ions in solution, isostructural to [AI(OH) ..r, suggested good suitability as a zeolitic agent. Very little is known about the mechanism of nucleation in zeolite crystallization. This is largely due to the complicated nature of the reaction medium and the absence of suitable tracer techniques. Two main proposals have been put forward: 1. Solid-phase transformation" 2. Liquid-phase transformation."

There are arguments for and again st both mechanisms, but it is proven that the [AI (OH )4r ion plays a very important role in the crystallization process. MeNicol and Pott 7 ,8 suggested that upon initial mixing of the gel tetrahedral sites are formed similar to the metal environment found in the product. Therefore the presence of soluble, tetrahedral [Fe0 4] 2- ions in the reaction mixture might encourage nucleation and incorporation of iron within a silicate framework.

EXPERIMENTAL Reagents Reagents used were: sodium hydroxide. pellets (Laboratory reagent, Scientific and Chemical Supplies Ltd.); iron(I1I) chloride (CPR, BDH); iron (III) nitrate 9-hydrate (99%, Analar, BDII); benzene (Analar,

0144-2449/96/$15.00 PII 50144·2449(96)00093-0

Synthesis of iron cancrinite: K. Latham et al.

II I

III

I I -

..... ·-- ..,-.-.1

B Figure 1 (a) XRD pattern of iron-substituted cancrinite prepared. compared with JCPDS standard 134-1761. (b) XRD pattern of aluminum cancrinite prepared, compared with JCPDS standard [34-1761.

BDH); methanol. absolute (Analar, BDH); ethanol, absolute (Analar, BDH); fumed silica (Cab-o-sil M5 BDH); aluminum powder (CPR, Scientific and Chemical Supplies Ltd.): and deionized water.

Preparation of sodium ferrate(VI) An attempt at the preparation of sodium ferrate, by the oxidation of iron (III) in aqueous alkali by chlorine was undertaken, adapted from a method for the preparation of potassium, rubidium. and caesium ferrates, outlined by Audette and Quail ." We were unable to isolate sodium ferrate(VI) as a solid , and therefore the sodium fcrrate(VI) solution was used in the zeolite synthesis . To prevent reduction of the reactive ferrate ion on addition to a zeolite gel, the sodium ferrate(VI) solution was introduced into a highly basic sodium aluminate solution, and then the silica slurry was added to this to form a homogeneous gel. Also. due to the oxidizing power offerrate(VI) with soluble organic substances, no templating agent was used .

Zeolite preparation A zeolite "A"-type gel was prepared by addition of aluminum powder (0.055 g) to the alkaline aqueous sodium ferrate mixture (60 crrr': 14.4 g of sodium hy-

. ~

~

a

"

e

droxide, 0.68 g of sodium ferrate(VI) , 50 cm ll of water) , followed by the addition of a corresponding amount of fumed silica (0.492 g). The resulting, fluid mixture contained a very large excess of sodium hydroxide and water and had the following molar composition: 43.9Na20 0.5Fe203 0.5Al203 2Si0 2 678H 20 The gel was placed in PTFE bottles (75 cm 3 capacity) and was heated to 100·C. Samples were removed after 1,3, and 7 days and were cooled to room temperature. The product, a beige solid , was collected by filtration or centrifugation, washed with distilled water, and dried at 40·C in air.

RESULTS AND CONCLUSIONS Analysis by X-ray diffraction (Philips 1710 X-ray d iffractometer using CuKa radiation) showed that cancrinite had been formed (Figure 1a). This sample was compared with a standard sample made "in house" in the absence of an iron source (FifJ.Ure 1b). The main XRD reflection occurred at 3.2440 A in the iron-containing sample, which corresponds to 3.2065 A in the standard. When both samples were indexed (hexagonal primitive), the iron-containing sample had a pnit cell volume of 721.84 All compared with 708.13 A3 for the pure aluminum-containing sample. An external quartz stanTable 1 Thermogravimetric analysis of cancrinite and ironsubstituted cancrinite

I ,

:!

Cancrinite

b

3.

llMlO

.

. ,t ,

~-I~

2000

\\ ...numbu I em

1000

soo

I

Figure 2 Infrared spectra of Is) canc rinite end (b) ironsubst ituted cancrinite, showing changes in intensity and resoJulion of main absorption peaks.

514 Zeolites 17:513-516, 1996

Peak temperature (OC) 58 283 579 626 752 786

tron-substituted cancrinite Weight loss (%)

1.24 5.44 0.25 1.29 0.24 0.74

Peak temperature (OC)

Weight loss (%)

65

8.91

552

1.91

785

0.42

Synthesis of iron cancrinite: K. Latham et al.

B

A

Figure 3 Electron micrographs of (a) cancrinite and (b) iron-substituted cancrinite.

dard (Permaquartz, General Electric Company) was used in the calibration of the X-ray diffractometer. The infra-red (Philips PU9624 FTLr.) spectra (Figure 2) are similar. No obvious shifts are observed in the peak positions of the iron-substituted sample, but changes in intensity are apparent, especially the main absorption bands at 1444, 1423, 1035, 991, and 964 cm" . The peak at 1118 cm" in the iron-eancrinite is also more clearly resolved. Thermogravimetric analysis (Mettler TG50 Thermobalance/Mettler TA3000 processor under nitrogen at a heating rate of 20 Kmin- l using -15 mg of the sample) showed a loss of fine features in the iron-eontaining sample (Table 1), and water loss occurred at a lower temperature. This indicates that the zeolitic water is less strongly bound. Structure collapse, as monitored by X-ray diffraction, occurred at similar temperatures -827°C for the aluminum cancrinite and -811 DC for the iron-containing cancrinite. The large number of individual weight losses occurring for the aluminum cancrinite is believed to be due to water associating with different aluminum sites. The low-temperature water is that which is simply occluded onto tile external surface of the material. The weight loss at 283 is believed to be due to occluded water within the pore system of the zeolite, while the higher temperature losses are due to the removal of associated water from the aluminum sites. The reason for the three small (-0.25%) high-temperature weight losses is unclear, but on repeated thermal analysis these steps were distinguishable. They must correspond to slight changes in the aluminum environment, which may be induced by the steam released during thermal gravimetric analysis. SEM (Philips 515) showed that the standard cancrinite contained hexagonal rod-like crystals of approximately 1 x 3 pm in size. The iron-substituted sample had a similar morphology, but the crystals were larger and also contained some amorphous material (Figure 3). The pure aluminum cancrinite and the ironcontaining cancrinite were analyzed by XRF (ARL 8410) chemical analysis, and the results are shown in DC

Table 2.

The XRF program used to determine the elemental compositions is based on 40 U.S. and Japanese certified geological standards, and is curve fitted with a best-fit polynomial. This program has a relative error of 2% associated with each determination. The values given above have not been "normalized" to 100%. Therefore , the totals for aluminum cancrinite add up to 99.8%, whereas the iron cancrinite totals add up to 101.2%. The Na20/ Al20S ratio for the aluminum cancrini te is 1:1.02, very close to the ideal of 1:1. The Na20/ (A1 20s + Fe20 S ) ratio for the iron cancrinite is 0.92:1, which is rather low on the sodium content. There are several possible reason for this: (a) (b) (c)

the lack of sensitivity for the light elements using XRF is well known ; the presence of a small amount of amorphous silica/alumina would produce a low relative sodium reading; some sodium has been removed by excessive washing.

As regards (b), there is no evidence from XRD that there is appreciable amorphous material present within the sample, however XRD is only sensitive to material contents of -5% and above. As regards (c) both the aluminum and iron cancrinites were washed in an identical manner. The iron cancrinite was magnetically analyzed using a Gouy balance (Newport Instruments Ltd.) comprising a cryostat control, electromagnet, and vacuum facilities. All weight readings were recorded to four decimal places at 298 K and a field strength of 7 k Gauss. The balance was calibrated using Hg[Co(SCN)4] ' The iron cancrinite produced a magnetic moment of 5.83 BM, which is within the range for tetrahedral iron. Cancrinites are well known for their ability to occlude salts within their cage-systems. IO However. the considerable movement to higher d-values of the main reflections in the XRD pattern does suggest incorporation of iron (III) into the zeolite framework. The thermal analysis results also indicate that water is less strongly bound in the iron-eontaining samples, and it is

Zeolites 17:513-516, 1996 515

Synthesis of iron eanerinite: K. Latham et al.

Table 2 XRF analysis of cancrinite and iron containing cancrinite Weight elemental oxides (%) Sample Aluminum cancrinite Iron cancrinite

REFERENCES

sio,

Al z0 3

Fez0 3

NazO

37.5

26.2

0.0

26.9

9.2

36.9

19.3

8.3

25.4

11.3

H2O

likely that iron (III) ions, in lattice sites, will have less attraction to polar molecules such as water, compared to Al~+, due to a lower charge-to-radius ratio.

ACKNOWLEDGMENTS We thank Dave Crane for scanning electron microscopy, Brian Bucknall for XRF analysis, and Dr. Jeff J. Cox for magnetic susceptibility measurements. KL. would like to acknowledge the University of Wolver-

516

Zeolites 17:513-516, 1996

hampton for the award of a research grant and Prof. Mike Adams for useful discussions.

1 Cotton. FA and Wilkinson. G. Advanced Inorganic Chemistry: A Comprehensive Text. 4th ed. Wiley, New York. 1980/ pp.765-766 2 Williams, D.H. and Riley. J.T.lnorganiea Chimica Acta 1974/ 8. 177-183 3 Barrer, R.M. and Cole, J.F. US Pat. 3,674,709 (1972) 4 Barrer, R.M. and Cole, J.F. J. Chern. Soc. (A) 1970, 15161531 5 Zhdanov, S.P. Adv. Chern. Ser. 1971/ 101,20 6 Breck, OW. and Flanigen, E.M. Proceedings of the 137th National Meeting of the American Chemical Society, Cleveland, Ohio, 1960 7 McNicol, B.D. and Pon, G.T. J. Chern. Soc., Chern. Commun; 1970, 438 8 McNicol, B.D. and Port, G.T. J. Catal. 1972,25,223-229 9 Audette, R.J. and Quail, J.W. Inorg. Chern. 1972. '1(8)/ 1904-1908 10 Szostak, R. Molecular Sieves; Principles of Synthesis end Identification, Van Nostrand Reinhold Catalysis Series. Van Nostrand Reinhold. New York. 1987