H.K. Beyer, H.G. Karge, I. Kiricsi and J.B. Nagy (Eds.) Catalysis by Microporous Materials Studies in Surface Science and Catalysis, Vol. 94 �9 1995 Elsevier Science B.V. All rights reserved. 317

Synthesis of Al-free Sn-containing molecular sieves of MFI, MEL and MTW types and their catalytic activity in oxidation reactions

Nawal Kishor Mal, Asim Bhaumik, Veda Ramaswamy, Anagha A. Belhekar and Arumugamangalam V. Ramaswamy

National Chemical Laboratory, Pune 411 008, India

Al-free Sn-silicalites with MFI, MEL and MTW structures have been synthesized hydrothermally and characterized by XRD, FT-IR, ll9Sn MAS-NMR and sorption techniques. The unit cell volume expansion depends on the manner Sn atoms are linked to the silicalite network. Framework IR spectra show an absorption at 970 cm -1 which is associated with Si-O-Sn vibration. SnOx units may be linked through an edge with SiO4 tetrahedra and can be described as structural defects. Sn-silicalites are active in the oxidation of phenol, toluene, m-cresol and m-xylene with aqueous H20 2 as oxidant leading to products of both aromatic hydroxylation and oxidation of the methyl substituent. The product distribution in general and the ability of Sn-MTW-silicalite to oxidize bulkier naphthalene and 2-methylnaphthalene suggest that the Sn 4 + ions are located within the channels of the molecular sieves.

1. INTRODUCTION

Substitution of tin in molecular sieve zeolites is expected to impart certain properties which are useful in their application as adsorbents [1], as ionic conductors [2] or as catalysts in several hydrocarbon conversion processes [3,4]. The post-synthesis procedures that employ either the chlorides or fluorides of Sn to react with parent zeolites (faujasites, ZSM-5, zeolite-L or omega) at high temperatures have claimed to substitute Sn for A1 in them [3,4], but lead invariably to loss of crystallinity of the zeolites. Exxon has recently claimed the synthesis of stannosilicotes under hydrothermal conditions in presence of alkali metals and A1 or Ga, where Sn 4+ ions are reportedly octahedrally coordinated [1]. Attempts to incorporate Sn ions into the pentasil structure directly during the process of hydrothermal synthesis have been reported [5-7]. However, substitution of Sn z~ + cation within the oxygen framework of MFI was not evidenced [6]. We have recently communicated the synthesis of Al-free. Sn-containing silicalite-1 (MFI) and silicalite-2 (MEL) and showed that some of the Sn 4 + ions could be in the framework positions [8,9]. We have also shown that these Sn-silicalites are catalytically active in oxidation and hydroxylation reactions using aqueous H20 2 similar to titano-silicates, TS-1 and TS-2, although the activity was lower.

In this communication, we describe briefly the synthesisof Sn-MFI, Sn-MEL and Sn-MTW silicalites, their characterization by XRD, IR and l l9Sn MAS-NMR spectroscopy and their activity in the oxidation of a few organic substrates in presence of aq. H20 2 in order to differentiate the medium pore (MFI and MEL) Sn-silicalites from the large pore MTW-type Sn-silicalite molecular sieves.

318

2. EXPERIMENTAL

2.1. Synthesis

The hydrothermal synthesis of Al-free tin-silicalites was carried out using gels of the following molar compositions: 1.0 SiO2 : x SnO2 : 0.13 -0.45 R + OH : 30 - 35 H20, where x - 0.00 to 0.03 (MFI), 0.04 (MEL) and 0.014 (MTW) and R + - tetrapropyl ammonium (20 % aq.) (MFI structure), tetrabutyl ammonium (40 % aq.) (MEL structure) or hexamethylene bis(benzyl dimethyl ammonium) (MTW structure). In a typical synthesis, 0.51 g of SnC14.5H20 (Loba Chemie, 98 %) was added to a solution of 21.3 g of tetraethyl orthosilicate (TEOS).(Aldrich, 98 %) under stirring. After 15 min, the alkali metal ion-flee organic template (R § OH-) was added under vigorous stirring. This mixture was stirred for I h before addition of water to give a clear solution which was stirred for another 30 min. In the case of MTW silicalite, 0.39 g NaOH was added and stirred for another 20 min. The homogeneous reaction mixture was charged into a 100 ml capacity stainless steel autoclave and heated at 433 K for 2 to 5 days for the crystallization to complete. After crystallization, the product was filtered, washed with deionised water, dried at 383 K and calcined at 773 K. The product yield was between 70 and 80 mass %. In the case of Sn-MTW, the Na + ions were exchanged for H + ions by usual procedure. For comparison, respective Sn- and Al-free silicalite samples were also prepared using the above procedures. These were subsequently impregnated with SnC14 solution and calcined.

2.2. Characterization

The possibility of incorporation of Sn into the silicalite framework was examined by means of powder XRD in a Rigaku (D-Max III-VC model) instrument using Cu Ka radiation and measuring the expansion of interplanar d-spacing corresponding to the major 20 intense XRD peaks in the 20 range, 5 - 60 ~ The samples were calcined and saturated overnight at 35 % relative humidity for XRD measurements.

The framework IR spectra were recorded in a Nicolet (60 SXB model) instrument using KBr pellet techn'ql ue. 1 lVSn MAS NMR s p ectra were obtained" at 111 .82 MHz on a Bruker MSL-300 NMR instrument. Typically around 3000 transients were signal averaged before Fourier transformation. The chemical shifts were referenced externally to tetramethyltin.

The bulk Si/Sn ratios of the calcined samples were obtained by XRF (Rigaku, model 3070) technique. The surface Si/Sn ratios were calculated from the integrated intensities of Si2s and Sn3d peaks of the XPS spectra (VG Scientific ESCA-3MK2 electron spectrometer) using A1 Ket x-ray source. A binding energy of 285 ev for C ls level was used as internal standard.

2.3. Catalytic Experiments

The oxidation of phenol and toluene was performed in a batch reactor at 348 and 353 K, using water and acetonitrile as solvent, respectively with a substrate to H20 2 mole ratio of 3. The oxidation of m-cresol, m-xylene, naphthalene and 2-methylnaphthalene was carried out in a stirred autoclave (Parr instruments, USA) of 300 ml capacity under autogenouspressure. Typically, 1.0 g of the catalyst and 5 g of the substrate in 20 g of acetonitrile/H20 (solvent) and appropriate quantity of aqueous H20 2 (26 % by wt.) (substrate to H202 of 3 mol) were placed in the reactor. After completion of the reaction (24 h), 25 g of acetone was added to the products, which were then separated from the catalyst by filtration and analysed by GC (HP 5880) using a capillary (cross-linked methylsilicon gum) column and flame ionization detector. The identity of some of the products was confirmed by GC-MS (Shimadzu, QP 200 A model).

319

rO * <

o

0 I 2

Sn A T O M S / U N I T CELL

Figure 1. Unit cell volume vs. Sn content in MFI(a), MEL(b) and MTW(c) silicalites.

Figure 2. Framework IR spectra of Sn- silicalites of MFI(a), MEL(b) and MTW(c) structures compared with Sn- impregnated silicalite-1 (d)and pure SnO2 (e).

I-- I--..

03 z ,<

15 %

| ~" I I �9

1300 IO(X~ 700 4 0 0

WAVE NUMBER ( cm - I )

3. RESULTS AND DISCUSSION

3 .1 . S y n t h e s i s , C r y s t a l l i z a t i o n a n d S t r u c t u r e

In the hydrothermal synthesis of the Sn-silicalites, the formation of a homogeneous gel containing the Si and Sn source is an important step. The uptake of Sn into the silicalite network depends among other things on the pH of the gel, which was around 12.3 for Sn-MFI and Sn-MEL and 11.7 for Sn-MTW. It is to be seen that the reaction between SnC14 and TEOS is complete before the addition of the organic base or NaOH. At such pH levels, hydrated Sn-hydroxide is completely dissolved forming a clear gel. The procedure described in the patent literature [7] for the synthesis of Sn-silicalite-1 (MFI) in fluoride medium (pH = 6.3) led to very large crystals of silicalilte-1 and it was doubtful if Sn was incorporated into the structure [9]. The scanning electron micrographs of our samples showed that the crystalline particles were of uniform size (0.2-0.5 pm) but much smaller than those of parent Sn-free silicalites (3-5 pm size).

The XRD profiles of the calcined Sn-silicalite samples showed them to be highly crystalline with no impurity phases in each case. Compared to the XRD patterns of Sn-free silicalites, the Sn-containing samples showed somewhat broader peaks, obviously due to small crystallite size. The unit cell volumes calculated after refinement of the peaks following the least square fit in each case show an increase with the number of Sn atoms per unit cell (Table 1). In none of the cases, however, this increase is equivalent to the theoretical v lu o 4+ o 4+ a es (Shannon ionic radii, 0.55 A for Sn and 0.26 A for Si ). The lower slope in all the three cases (Fig 1) indicate that either onlv a oart of the total Sn 4 + ions Cabout

�9 " ~ 4 + one fifth) are in framework positions and/or that most Sn ions assume coordinations other than tetrahedral. It is possible that Sn 4 + ions are incorporated in the edge-sharing

320

Table 1 Composition and physico-chemical characteristics of Sn-silicalite samples.

Sample

Si/Sn (mole Sorption ll9Sn Sur- Meso- ratio) capacity a, NMR b face pore

(wt.%) area area

Gel Product c (V~) Chem XPS H20 Cyclo n- 6 (m2g - 1) (m2g - 1) Anal. hexane Hexane (ppm)

Sn-Sil-1 Sn-Sil-1 Sn-Sil-1 Sn-Sil-2 Sn-Sil-2 Sn-Sil-2 Sn-ZSM-12 Sn-ZSM-12

33 29 - 5371 8.0 6.0 16.5 -685 50 47 53 5365 7.5 4.8 16.0 -693 133 85 77 5346 6.5 4.2 13.5 - 50 49 44 5360 8.5 11.3 14.0 -739 70 63 65 5358 7.5 11.0 13.7 -705 100 102 98 5349 7.2 10.5 13.5 -740 75 73 67 1453 8.9 12.2 9.8 - 100 98 95 1447 8.3 12.0 9.6 -

177 177 1435 7.5 11.1 8.9 - - - 5345 4.8 4.0 12.5 - - - 5345 4.0 8.0 12.4 - - - 1423 4.9 10.5 8.2 -

Sn-Z~M-12 180 Sil-1 u Sil-2 d _ ZSM_12 d _

527 42 522 38 500 45 557 30 554 35 506 56 321 50 310 38 301 45 384 11 387 7 280 15

aGravimetric (Cahn balance) adsorption at P/Po = 0.5 and at 298 K. bChemical shift with respect to Me4Sn. CCa]cined products; bulk composition by chemical analysis and surface composition by XPS. uSn-free silicalites.

or corner-sharing positions of the silicalite network, which will account for the linear increase in the unit cell volumes observed upto 3 atoms of Sn per unit cell (MFI and MEL). In the cgse of titano-silicates (TS-1), for example, Tuel and Ben Tarrit [10] have shown that Ti 4+ ions could occupy such positions (other than isomorphous substitution) depending on the source of Ti (Ti alkoxides) and Si used in the hydrothermal synthesis. Bigger cations such as Ti 4 + and Sn 4 + (compared to Si 4 + ) may expand its coordination to a five- or six-fold one, on interaction with one or two more li~ands [11]. In our samples, there is a strong indication of octahedral coordination for Sn 4 +~ ions from the MAS-NMR studies [9].

3.2. Spectral Characterization

Despite the high Si/Sn ratios of the samples, 119Sn MAS-NMR signals are detected owing to large sensitivity associated with the spin 1/2 of 119Sn nucleus. The signals are located in the range, -700 to -750 ppm for all the samples (Table 1). Although octahedrally coordinated Sn in pure SiO2 has a chemical shift of-604 ppm [12], in many ternary tin oxides the octahedral tin environment resonates in the chemical shift range of-450 to -750 ppm. The observed chemical shifts in our samples are in overlap with the reported range for octahedral tin. It is more probable that Sn 4 + is incorporated at or very close to the defect (silanol) sites [9].

The chemical analysis of the gel (by AAS) and the product (by XRF) indicate that the Si/Sn ratios are fairly well maintained (Table 1) in the product after crystallization, the Sn-uptaken from the gel in most of the samples being close to 90 % or more. A fairly good correspondence between the bulk and the surface Si/Sn ratio (from XPS), on the other hand, indicates fairly uniform distribution of Sn 4 + ions throughout the bulk of the samples (Table 1).

321

Table 2 Hydroxylation of phenol on Sn-Silicalite molecular sieves a.

Catalyst Sn-Sil- 1 Sn-Sil-2 Sn-ZSM- 12 Si/Sn mole ratio 50 70 50 70 73 100

H 20 2 efficiency b. 68.6 59.3 63.3 54.2 51.6 42.3 Product distribution, mole % p-Benzoquinone 2.3 2.5 0.3 0.2 2.8 3.0 Catechol 59.2 60.0 52.1 51.8 56.1 56.6 Hydroquinone 34.5 32.2 42.4 43.2 37.8 37.3 Tars 4.0 4.3 5.2 4.8 3.3 3.1 o-/p- ratio 1.6 1.7 1.2 1.2 1.4 1.4

aReaction conditions: Catalyst/Phenol = 10 g mol-1; Phenol/H202(mole)= 3; H20(solvent)/Phenol(mole ) = 20; Temp. = 348 K; Time = 24 h; Slow addition of H20 2 over a period of 1 h in batch reactor. ~ (mole %) in relation to the initial concentration of H20 2.

The FTIR spectra of the calcined samples show that all samples are highly crystalline (Fig. 2). Evidence for the possible Si-O-Sn linkages in all the samples is suggested by the presence of an absorption band at around 970 cm- ~ in the IR spectra (curves, a to c), which is similar to the observation in silastannoxanes of the type, R3Sn-O-SiR 3 [13]. A similar observation for Ti- and V-silicalites has been attributed to Si-O-M vibrations, although other interpretations for the origin of the 960 cm -1 band in TS-1 have been proposed [11,14]. In the Sn-impregnated silicalite-2 sample, no such vibration is noticed (curve d). Pure SiO2 shows absorptions due essentially to Sn-O stretching vibrations (curve e).

3.3. Texture and Sorption properties

The surface areas determined from the N 2 adsorptign i~otherms in the low partial pressure region (upt9 P~/Po = 0.05) are in the range of 500 m z g-I for Sn-MFI and Sn-MEL samples and 300 mZg -~ for Sn-MTW samples (Table 1). It is estimated that meso pore areas (determined form the t-plots at higher P/Po values) contribute roughly to 10% of the total area. The amount of H20, cyclohexane and n-hexane adsorbed by the samples at 298 K and at P/Po of 0.5 are included in Table 1. From the amount of H 2 0 adsorbed, it may be concluded that the Sn-silicalites are more hydrophilic than the parent Sn-free silicalites. The sorption capacities for n-hexane and cyclohexane in all the samples show that the micropore volumes are maintained and that occluded SnO2 type of species may not be present in them.

Table 3 Oxidation of toluene on different Sn-Silicalite molecular sieves a.

Catalyst Sn-Sil- 1 Sn-Sil-2 Sn-ZSM- 12 Si/Sn mole ratio 50 70 50 70 73 100

H20 2 efficiency, mole % 42.3 39.4 38.6 36.4 34.2 29.6 Product distribution, mole % Benzyl alcohol 6.3 6.8 10.2 11.5 14.2 15.5 Benzaldehyde 77.4 76.9 72.2 71.4 67.9 66.7 o-Cresol 3.7 3.5 5.0 4.1 3.1 3.7 p-Cresol 6.8 6.3 8.2 8.8 9.0 8.7 m-Cresol 2.6 2.5 1.6 1.2 0.8 0.6 Others 3.2 4.0 2.8 3.0 5.0 4.8

aReaction conditions: Catalyst/Toluene = 20 g mol'l; Toluene/H202(mole) = 3; Solvent (acetonitrile) = 20 g; Temp. = 353 K; Time = 24 h; (batch reactor).

322

Table 4 Oxidation of meta-Cresol on Sn-Silicalite molecular sieves a.

Catalyst Sn-Sil- 1 Sn-Sil-2 Sn-ZSM- 12 Si/Sn mole ratio 70 70 73

H 2 0 2 efficiency, mole % Product distribution, mole % 2-Methylhydroquinone 4-Methylcatechol 3-Hydroxybenzyl alcohol 3-Hydroxybenzaldehyde Others

69.3 63.5 73.5

35.3 38.2 37.0 24.8 23.4 25.0 4.3 9.4 8.5

31.3 25.0 26.3 4.3 4.0 3.2

aReaction conditions: Catalyst/m-cresol = 20 g mo1-1 ; m-Cresol/H202(mole ) = 3; H20:acetonitrile(3:l)/m-cresol(mole ) = 20; Temp. = 353 K; Time = 24 h; Reaction carried out in Parr reactor.

3.4. Catalytic activity

Like the titano-silicalites and the vanadium silicalites reported earlier, the Sn-silicalites are catalytically active in the oxidation reactions with aq. H20 2 [7-9]. A comparative account of the catalytic efficiency of the three Sn-silicalites with Si/Sn = 50 and 70 each in the hydroxylation of phenol to give dihydroxybenzenes is given Table 2. On the basis of H202, the selectivity to dihydroxybenzenes is the highest for Sn-Sil-1 (69 %). The selectivities to catechol and hydroquinone changed with reaction time and at the end of the reaction (24 h), the products were composed of more catechol than hydroquinone in all the cases, the o-/p- ratios being 1.6, 1.2 and 1.4, respectively for Sn-Sil-1, Sn-Sil-2 and Sn-ZSM-12. Under similar conditions, a catechol to hydroquinone ratio of 0.9 to 1.1 has been observed on TS-1 and TS-2 earlier [15]. These results indicate that well-dispersed Sn 4 + ions which are probably located within the channels are responsible for the catalytic activity. The low efficiency of Sn-Sil samples, in general, compared to TS-1 could be explained on the basis of rapid decomposition o fH20 2 on Sn sites. Also, these Sn-silicalites are mildly acidic, as seen from their ability to dehydrate cyclohexanol to cyclohexene at 453 K.

Table 3 compares our results on the oxidation of toluene over the Sn-silicalite samples. The Sn-samples are active in this reaction (39.4, 36.4 and 34.2 mol % H20 2 efficiency in 24 h for samples with Si/Sn ratios of 70). Both the hydroxylation of the aromatic nucleus to give cresols and the oxidation of the methyl substitutent to give benzyl alcohol and benzaldehyde take place simultaneously on the Sn-silicalites. Based on the product distribution, it can be seen that the rate of the oxidation of the methyl substituent is about 6 times faster than the rate of aromatic hydroxylation on all the samples. After 24 h, the concentration ofbenzaldehyde is the highest in the product. In this respect, the Sn-silicalite molecular sieves are more similar to the V-silicalites, VS-2 than the Ti-silicalites, TS-1 or TS-2 [16].

The oxidation of m-cresol was carried out in Parr autoclave at 353 K using a 3" 1 mixture of H 2 0 and acetonitrile as solvent and Sn-silicalites with Si/Sn ratio of 70 as catalysts. A slightly higher efficiency for H20 2 is seen with Sn-ZSM-12 sample (Table 4). The dihydroxylated products, viz., 2-methylhydroquinone and 4-methylcatechol are found to be in excess over the products of side chain oxidation, viz., 3-hydroxybenzyl alcohol and the aldehyde in the product mixture. The aromatic hydroxylation on Sn-silicalites may follow an ionic mechanism as both the -CH 3 and -OH groups in m-cresol are favourably placed for electrophilic substitution reaction. Interestingly, the product distribution on all the three Sn-molecular sieves is almost similar. This shows that in all the three types, the Sn 4 + ions are dispersed uniformly and possess identical catalytic property due to similar environment around them.

323

Table 5 Oxidation of meta-Xylene on Sn-Silicalite molecular sieves a.

Catalyst Sn-Sil- 1 Sn-Sil-2 Sn-ZSM- 12 Si/Sn mole ratio 70 70 73

H 2 0 2 efficiency, mole % Product distribution, mole % 3-Methylbenzyl alcohol 3-Methylbenzaldehyde 2,4-Dimethylphenol 2,6-Dimethylphenol 3,5-Dimethylphenol Others

65.6 57.9 68.5

13.2 23.2 21.0 57.0 47.2 49.0 15.0 14.3 13.4 6.3 8.7 6.4 5.5 3.4 5.2 3.0 3.2 5.0

aReaction conditions: Catalyst/m-xylene = 20 g mol-1; m-Xylene/H202(mole ) = 3; Solvent (acetonitrile)/m-xylene(mole) = 20; Temp. = 353 K; Time = 24 h; Reaction carried out in Parr autoclave.

Under similar conditions, the oxidation of m-xylene on the three Sn-silicalites shows similar conversions and H 2 0 2 efficiencies, but the product distribution is different (Table 5). The products from the oxidation of the -CH 3 group (3-methylbenzyl alcohol and aldehyde) are the major components (about 70%) and the phenolic products (2,4-, 2,6- and 3,5-dimethylphenols) constitute 26% of the products after 24 h of reaction. Such a selectivity could result from the greater possibility of either of the two methyl groups undergoing oxyfunctionalization than the aromatic hydroxylation over Sn sites.

The oxidation of naphthalene and 2-methylnaphthalene was carried out on the Sn-silicalites with Si/Sn = 70 and the results are summarised in Table 6. From the H 2 0 2 yield it is clear that Sn-ZSM-12 is more active than the medium pore Sn-silicalites, due probably to better diffusivity of the reactants through the large pore MTW channels. The hydroxylation of naphthalene leads primarily to the formation of 1- and 2-naphthol and the oxidation of 1-naphthol to form of 1,4-naphthaquinone. These three products constitute 97% of the product selectivity. In the oxidation of 2-methylnaphthalene, a slightly higher H 2 0 2 efficiency is recorded and about 50% of the products result from the aromatic ring hydroxylation (three isomers of mono-hydroxylated 2-methylnaphthalene). The oxidation of the methyl substituent gives 2-naphthalene methanol and 2-naphthaldehyde (47 %). These results demonstrate that well-dispersed Sn 4+ ions present in the large pores of the MTW structure are responsible for the oxidation of the bulkier substrates.

Table 6 Oxidation of Naphthalene (A) and 2-Methylnaphthalene (B) on Sn-Silicalite molecular sieves a.

Catalyst Sn-Sil- 1 Sn-Sil-2 Sn-ZSM- 12 Si/Sn mole ratio 70 70 73 Substrate A B A B A B

H 2 0 2 efficiency, mole % 8.2 7.3 Product distribution, mole % Products of aromatic hydroxylation 97.2 44.9 96.9 2-naphthalene methanol - 13.7 - 2-naphthaldehyde - 38.9 - Others 2.8 2.5 3.1

10.8 6.1 26.8 31.6

45.5 97.0 49.6 15.5 - 12.2 37.0 - 35.0 2.0 3.0 3.2

aReaction conditions: Catalyst/Substrate - 20 g mo1-1 ; Substrate/H202(mole) - 3; Solvent (acetonitrile) - 20 g; Temp. - 353 K; Time - 24 h; Reaction carried out in Parr autoclave.

324

The origin of the catalytic oxidative activity of the the Sn-silicalites is not clear at the moment. It may be due to the reduction of isolated Sn 4 + to Sn 2 +, which is then oxidised back with H20 2. Also, many hydroperoxides of tin have been known from the action of H20 2 upon solutions of Sn 2 § and Sn 4 +. With our Sn-silicalites, however, there was no evidence for the dissolution of Sn under the reaction conditions as they have been regenerated after the reaction and reused several times without significant loss of catalytic activity. Surface tin hydroperoxides may be the active species but further detailed studies are required before possible mechanisms of oxidation involving Sn could be discussed.

4. CONCLUSIONS

Sn-silicalites of MFI, MEL and MTW structures with Si/Sn > 30 have been synthesized hydrothemally under basic conditions. The unit cell volume expansion in each case, though linear with respect to Sn content (upto 3 Sn per unit cell in MFI and MEL silicalites), does not correspond to theoretical T-atom substitution by Sn 4 + ions. The well-dispersed SnOx units can be described as structural defects with octahedral coordination and are active in the oxidation of a number of organic substrates (phenol, toluene, m-cresol and m-xylene) with aqueous H20 2. These are similar to vanadium silicalites (VS-1 and VS-2), as both hydroxylation of the aromatic nucleus and the oxidation of the alkyl substituent are catalysed. Due to the presence of Sn 4 + in large pores, Sn-ZSM- 12 sample is able to oxidize bulkier naphthalene and 2-methylnaphthalene more effectively than the medium pore Sn-MFI and Sn-MEL silicalites. Acknowledgement

We thank Dr. S. Badrinarayanan for XPS and Dr. S. Ganapathy for NMR spectra. Analytical help from Dr. S.V. Awate (XRF) and Mr. S.P. Mirajkar (GC-MS) is gratefully acknowledged. Two of us (NKM and AB) are grateful to CSIR, New Delhi for research fellowships. REFERENCES 1. US Patent No. 5 192 519 (1993). 2. I.G.K. Andersen, E.K. Andersen, N. Knudsen and E. Skou, Solid State

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