The Chemistry and Catalytic Properties of Transition Metal Oxyanions in Sodalite Cages

D.M. Bibby,C.D.Chang,R.F. Howe and S. Yurchak (Editors),Methane Conversion 0 1988 Elsevier Science PublishersB.V.,Amsterdam - Printed in The Netherlands

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THE CHEMISTRY AND CATALYTIC PROPERTIES OF TRANSITION METAL OXYANIONS IN SODALITE CAGES

L.M. MORONEY, S. SHANMUGAM and A.G. LANGDON Chemistry Department, University of Waikato, Hamilton, New Zealand ABSTRACT The loading of zeolite cages with catalytically active species provides a strategy for the modification of zeolite properties and the preparation of multifunctional catalysts. Hydroxy-sodalite has been used to study the loading of sodalite cages with chromate, molybdate and tungstate ions by a dry salt high temperature reaction. The resulting noseans were used as model systems for examining some of the properties of the occluded oxyanions. INTRODUCTION The widespread application of zeolites as catalysts has directed attention towards enhancing and extending catalytic activity through the introduction of catalytically active metal species principally by cation exchange with lattice cations. In zeolites such as the X, Y and A type zeolites, the sodalite cages provide possible sites for accommodating reactive metal species. It has long been known that sodalites with cages containing up to four water molecules, two NaOH molecules, one molecule of monovalent salts and one molecule of divalent salts shared between two cages, can be prepared by hydrothermal synthesis (ref, 1).

However except for a limited number of special

cases (refs. 2-3) it does not appear that salt occlusion by the sodalite cages of zeolites can be achieved during hydrothermal synthesis (ref. 4 ) .

Even

for the case of feldspathoid synthesis, the hydrothermal reaction involving the salts of transition metal oxyanions such as chromate, tungstate and molybdate yields different crystalline phases (sodalite and cancrinite) of low salt loading (refs. 5-6).

An alternative route to the occlusion of salts in zeolite systems is by means of high temperature reactions involving previously imbibed salts. This procedure has been successfully employed to fill the zeolite cages of X, Y and A zeolites with anions such as chloride, bromide, iodide, nitrate and chlorate (refs. 4,7).

It offered a possible means of occluding the oxyanions of trans-

ition metals such as Cr, Mo and W in the sodalite cages of hydroxy-sodalite and zeolites.

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METHODS AND MATERIALS Hydroxy-sodalite was prepared from metakaolin in 4 m o l 1-' NaOH at 8 O o C . High temperature reactions were carried out in platinum crucibles, in contact with air and at atmospheric pressure.

Products were characterised by X-ray

diffraction (XRD) and the peak heights were used to obtain semi-quantitative data for the amounts of the crystalline phases present.

Oxidation/reduction

chemistry was studied using a vacuum line specially constructed to follow gas adsorption. RESULTS AND DISCUSSION Conversion of Hydoxy-Sodalite to Salt Loaded Noseans Salt loaded noseans were prepared by a high temperature reaction between excess salt and hydroxy-sodalite (ref. 8 ) : Na,Al6Si,O2,.4NaOH

+

Na,X

+.

Na6A1,Si,0,,.Na2X

+ Na,O +

2H20

where X = Cr0,2-.Mo0,2-,W0,2-. Nosean has a cage structure very similar to that of sodalite.

It can be

considered to be derived from sodalite by flattening the A1-0-Si bond angle to enlarge the sodalite cage.

Sodalite and nosean have common XRD peaks

corresponding to d = 6 . 2 4

and d = 3.71 A but because nosean does not exhibit

the systematic extinctions that occur in the XRD pattern of sodalite, it was possible to monitor both the total crystallinity of the sodalite/nosean mixture and the growth of nosean during the conversion reaction. Intermediate formation of nephiline, when it occurred, was monitored by the XRD line corresponding to d = 4.17

A.

Preliminary differential thermal analysis (DTA) experiments indicated that the melting points of the salts were depressed in the reaction mixture.

For

example, the melting point of Na2Cr0, in the sodalite/Na2Cr0, reaction mixture. was depressed by 22'C

to 77OoC. From studies at temperatures above and below

the effective melting points of the salts it was clear that whereas the reaction from sodalite to nosean at temperatures above the melting point was accompanied by significant initial structural collapse, the conversion at temperatures below the melting point was effected with little loss of crystallinity. The observation of increases in product nosean intensities without concomitant decreases of any other crystalline phase indicated the possibility that an amorphous phase was formed during the reaction.

However

these increases could have been due at least partly to the gradual improvement of the crystallinity ofthe already formed product phase.

From these and other

data (refs. 8-9) the following reaction scheme was devised for the high

temperature reaction: OH-Sodalite/Na,CrO, (1) J.

metastable expanded OH-Sodalite

Cr0,-Nosean At 900°C the rates of reactions 2 , 3 and 4 were very much increased. The rate of reaction 6 at 75OoC and 8OO0C were comparable but reaction 2 was much slower than reaction 6 at 75OoC. Direct conversion of sodalite to nosean is favoured by keeping the reaction temperature below the depressed melting point of the salt.

It would appear that when the loading of zeolite sodalite cages

is attempted, best results for high melting point salts can be expected if the reaction temperature is kept below the effective melting point of the salt in the zeolite system. Aspects of the Chemistry of Occluded Metal Oxyanions Although feldspathoid structures are generally thought to be insufficiently porous to find widespread catalytic applications, the reactions of the occluded oxyanions are of catalytic interest. Previous work with other systems has shown that occlusion affects the properties of both the encapsulated species and the lattice itself (refs. 1,6,10,11). (i) Thermal Stability. While chromate-nosean is stable in air and under vacuum at temperatures up to 750°C, molybdate- and tungstate-noseans appear to undergo partial decomposition under vacuum to produce pale blue-grey colours.

No measurable evolution of gas was observed however. (ii) Reduction with H,(g).

Reduction of chromate-nosean with H,(g) at

atmospheric pressure started above about 3OOOC and appeared to reach completion after about 1 hour at 48OoC. The H,(a)

consumed indicated that the green

colour of product formed was due to Cr,O,.

The nosean structure was found to

remain intact until virtually all of the chromate had been reduced whereupon a dispersion of Cr,03 in nephilene of low crystallinity was formed. For reactions in which the rate of H2(g) consumption was studied, it was found that the rate of reaction'was pressure dependent and after a temperature dependent induction period,varied initially with the square root of time as might be expected for a diffusion controlled reaction.

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The molybdate- and tungstate-noseans were much more stable than chromatenosean under reducing conditions. This is consistent with the properties of the pure salts. Sodium chromate is relatively easily reduced to oxidation state I11 whereas molybdates and tungstates tend to form polymeric 'bronzes' with oxidation state between V and VI.

THe formation of this type of compound

is not possible when the oxyanions are separated in cages. The amount of H,(g) adsorbed by molybdate-nosean at 800°C was sufficient to account for less than 14% reduction of the oxyanion to the V oxidation state.

(iii) Reoxidation with O,(g). noseans with O,(g)

Quantitative reoxidation of reduced chromate-

was possible providing the reduction step had not resulted

in the destruction of the lattice.

If s o , reoxidation produced a mixture of

chromate-nosean, nephilene and free Na,CrO,. (iv) ESR Studies. Iron-free chromate-nosean necessary for these studies was prepared from iron-free Al(OH),,

H,SiO,, NaOH and Na,CrO,

(ref. 9 ) .

ESR

studies of the products formed during reduction and oxidation reactions gave two discrete signals with g values of 1.987

*

0.002 and 1.974

f

0.002

consistent with Cr(V) and Cr(II1) species. The line widths observed were as expected for the relatively dilute dispersion of Cr species in the aluminosilicate matrix.

Semi-quantitative data for the reduction and reoxidation

reactions were obtained by plotting A/GM (where A is the peak to peak first derivative signal amplitude, G is the gain and M is the amplitude modulation) versus time. These data indicated a sequential nature of the oxidation/ reduction reactions. The ESR experiment also provided a useful means of monitoring structural changes in the reduced noseans.

ESR line broadening and

reduced amplitude were associated with l o s s of X-ray crystallinity. (v) Exchange Reactions.

The sodium cations of the salt loaded noseans were

exchangeable with other simple cations.

In the case of NHb+ exchange, the

thermal stability of the encapsulated oxyanions was markedly reduced. (vi) Catalytic Activity.

Preliminary studies have shown that the nosean

systems with occluded transition metal oxyanions are active oxidation catalysts for reactions involving small molecules. CONCLUSIONS Studies of high temperature, dry salt reactions with hydroxy-sodalite have provided useful insights into the behaviour of zeolites when loaded with high melting point salts and reacted at elevated temperatures. Such reactions at temperatures above the effective melting point of the salt are likely to lead to the formation of both amorphous aluminosilicate phases and crystalline product phases such as nephilene and nosean. The salt loaded nosean systems and their cation exchanged forms provide convenient model systems for investigating the chemistry of metal oxyanions in

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sodalite cages.

These systems also allow a means by which novel oxidation

states in a highly dispersed form may be prepared. REFERENCES R.M. Barrer and J.F. Cole, J. Chem. SOC. (A), (1970) 1516. G.H. Khul, Advan. Chem. Ser., 101 (1971) 75. R.M. Barrer and E.F. Freund, J. Chem. S O C . , Dalton Trans., (1974) 1049. J.A. Rabo, in J.A. Rabo (Editor), Zeolite Chemistry and Catalysis, A.C.S. Monograph No. 171, 1976, pp. 332-349. 5 R.M. Barrer and A.G. Langdon, personal communication. 6 R.M. Barrer, J.F. Cole and H. Villiger, J. Chem. SOC. (A), (1970) 1523. 7 J.A. Rabo, M.L. Poutsma and G.W. Skeels, in J.W. Hightower (Editor), Proc. Inter. Congr. Catal. 5th, Miami Beach, 1972, North Holland Publishing Co. Amsterdam, 1973, pp. 1353-1363. 8 L.M. Moroney, M.Sc. Thesis, University of Waikato, 1978. 9 S . Shanmugam, M.Sc. Thesis, University of Waikato, 1983. 10 R.M. Barrer and C. Marcilly, J. Chem. SOC. (A), (1970) 2735. 11 R.M. Barrer, E.A. Daniels and G.A. Madigan, J. Chem. SOC., Dalton Trans., 1 2 3 4

(1976) 1805.