synthesis and characterization of the novel tectosilicate sigma-2

Synthesis and characterization of the novel tectosilicate Sigma-2

Allan Stewart ICI Chemicals and Polymers Limited, Middlesbrough, Cleveland TS6 8JE, UK

Sigma-2, a novel tectosilicate phase, has been synthesized using 1-adamantanamine and characterized using X-ray powder diffraction, i.r. spectroscopy, electron microscopy and diffraction, and elemental analysis. Thermogravimetric analysis was used to study the removal of organic species that were occluded during~synthesis. The results are considered in the light of details of the structure, which was recently solved from powder diffraction studies by McCusker. Sigma-2 in its pure silica form may be designated a clathrasil with large cases and tight 6-membered ring windows, the structure being composed of previously unknown nonasil and eikosasil cages.

Keywords: Sigma-2; clathrasil; synthesis

I N T R O D U C T I O N

Recently, several novel silica-only tectosilicates, designated clathrasils, x have been synthesized and reported by Gies et al. 2-6 at Kiel. Examples include dodecasil 1H (Ref. 2), dodecasil 3C (Ref. 3), nonasil (Ref. 4), and deca-dodecasil 3R (Ref. 5), the names indicating the cage types from which the individual framework structures are buih. 6 Clathrasils have been described as clathrate compounds with 3- dimensional 4-connected host frameworks of silica, a class of porous tectosilicates with no exchange properties for the guest species.

In general, the method employed in preparation is one in which an aqueous silicate solution, produced by hydrolysis of an alkoxysilane, is reacted with an amine or other potential "guest" molecules until crystals are formed. In these crystals, the framework structure acts as "host" to the "guest" species. When" the "windows" of the polyhedral cavities or cages are too small to allow passage of the encaged "guest" species or their decomposition products, the silica materials have been called clathrasils.

A recent report from this laboratory 7 has shown how Sigma-l, the aluminosilicate analogue of deca- d o d e c a s i l 3R, may be p r e p a r e d u s ing 1- adamantanamine (AN) in the system, N a 2 0 - A N - A120~--SiO2-H20. The nature of the material when calcined is that of a zeolite with ion-exchange properties and selectivity in adsorption of small molecules and exclusion of larger ones. In this paper, the

Address reprint requests to Dr. Stewart at ICI Chemicals and Polymers Ltd., Research and Technology Department, P.O. Box 90, Wilton, Middlesbrough, Cleveland TS6 8JE, England. Received 31 August 1988; accepted 19 September 1988.

© 1989 Butterworth Publishers

synthesis and characterization of Sigma-2, another new tectosilicate, is reported; this material was also prepared using AN but under different reaction conditions. Techniques conventional to the study of zeolites were applied in examination of the properties of Sigma-2 materials.

E X P E R I M E N T A L

Syntheses were generally carried Out under auto- geneous conditions at 180°C and 500 rpm in stirred stainless-steel autoclaves, using colloidal silica "Syton X30" (Monsanto Chemical Co.) or "Nalfloc 1034A" (Nalfloc Ltd.), potassium hydroxide, sodium hydroxide, aluminium wire and sodium aluminate (BDH Chemicals Ltd.), and AN (Aldrich Chemicals Co. Ltd.). Products were removed, filtered, washed, and dried at 110°C. If conditions other than the above were employed, the differences are described in the text.

Samples were examined by conventional elemental analysis techniques, by scanning electron microscopy (Hitachi 580b electron microscope), X-ray diffraction (Philips APD 1700 automated XRD system using CuK~ radiat ion), e lect ron dif f ract ion (Philips EM400T electron .microscope), and i.r. spectroscopy (Perkin Elmer 580 spectrometer);. Thermogra- vimetric analysis (t.g.a.) and differential thermal analysis (d.t.a.) were done simultaneously on a Stan- ton Redcroft STA 781 instrument.

Calcinations were performed on samples of differ- ing aluminium contents using a muffle furnace and a range of temperatures and times. Attempts were made to ion exchange calcined materials containing aluminium, and sorptive properties were studied using a Sartorius Model 4433 vacuum microbalance.

140 ZEOLITES, 1989, Vol 9, March

Novel tectosilicate Sigma-2: A. Stewart

R E S U L T S

Synthesis and composition Sigma-2 has been made in pure form from both Na

and K systems (Table 1). From the Na system, Na20-AN-AI2Oa--SiO2-H20, several aluminosilicate phases may be prepared pure or in admixture, the relative amounts depending primarily on the Na20 and A1203 levels relative to SiO~. The effect of the SiO2/Al2Oa ratio on the products of synthesis at 180°C was that Sigma-1, the aluminosilicate version of deca-dodecasil 3R, and Nu-3, the high-silica levynite- type zeolite, s were formed at progressively lower ratios, while Sigma-2 normally resulted at high ratios when Na20:AN:SiO2:H20 were kept relatively con- st-ant at 3:20:60:2400. When the SIO2/A1203 ratio was high, exceeding 90, Sigma-2 became the preferred product. However, at the same A1 level, an increase in the Na20 level to 7 Na20:60 SiO2 caused Sigma-1 to be favored over Sigma-2. With SiO2/Al2Os = 60 and SiO2/Na20 = 20, the use of a higher temperature gave virtually pure Sigma-2 instead of Sigma-1.

For K-containing systems_ with SiO2/K20 ratios of about 20 where K was the predominating, if not the sole, alkali metal, Sigma-2 could generally be prepared in pure form with slightly higher AI levels in the reaction mixtures and more AI in the crystalline products (Table 1).

Since the synthesis mixtures containing AN have produced several phases, a much more complete description of the effect of synthesis variables on the products formed will be given in a later paper. 9

The crystalline products, which were identified as the novel phase Sigma-2, had both alkali metal and organic species within the tectosilicate frameworks. The synthesis products, washed and dried at 110*C, were analyzed and found to contain some 7-10 wt% C and 0.7-1.1 wt% N, in relative ratios equivalent to those for AN itself, suggesting that the organic had not undergone decomposition. Sodium systems gave Sigma-2 products with about 0.1-0.25 wt% Na, while those predominately using K as the alkali metal had about 0.15-0.45 wt% K. The level of alkali metal was not obviously influenced by the AI content of the

Sigma-2 products. Since it has also been established by X3C MAS n.m.r, that AN remained intact in the products, ]° typical Sigma-2 samples in a washed and dried form would contain about 1 AN for every 15 T-atoms and about 1 alkali metal atom for every 150 T-atoms. The significance of these values will be discussed later.

Characterization of Sigma-2 X-ray diffraction

The XRD pattern of Sigma-2 (Figure 1) suggested that its structure was new. While Sigma-2 can be prepared in forms containing significant levels of aluminium, the XRD pattern is not substantially changed by its incorporation, as is obvious from a comparison of Figure la and b. The resolution of the various peaks may change, generally being better.for large-crystal samples (as would be expected).

Scanning electron microscopy (SEM) SEM examination revealed that Sigma-2 was com-

posed essentially of discrete polyhedral crystals, muhifaceted (often with 12 facets) with slightly rounded edges. Representative electron micrographs are shown in Figure 2. The size of the crystals may vary from small to those of 10 Ixm and larger, depending on the conditions of synthesis, Some of the polyhedra are truncated; this is more evident with some samples than with others.

Electron diffraction studies Electron diffraction work showed Sigma-2 to have

the unit cell parameters a - b = 1.01 _+ 0.01 nm and c = 3.43 _+ 0.03 nm. Forbidden reflections indicated that possible space groups were I4Jamd, /42d , and I41md. Diffraction patterns are shOwn in Figure 3a and b.

I.r. spectroscopy The i.r. spectrum of "as-made" Sigma-2 (SiO2/

A1203 > 1000) given in Figure 4 is typical of a tectosilicate with intense T - O stretching modes at about 1100 cm -] and bending modes at about 500

Table 1 Synthesis of Sigma-2 and other phases from the system, xNa20:yK20:20 AN:zAI203:60 SIO2:2400 H20, stirred at 180°C

Reaction mixture SiO2/AI203 ratio

Prep no. x y z Products formed Reaction mixture Product

1 3 0 0 Sigma-2 oo > 1000 2 0 3 0 Sigma-2 oo > 1000 3 a 3 0 0 Sigma-2 oo > 1000 4 3 0 0.6 Sigma-2 b 100 92 5 0.8 3 0.6 Sigma-2 100 91 6 3 0 1 Sigma-1 60 55 7 0.8 2.8 1 Sigma-2 (95%) Sigma-1 (5%) 60 48 8 3 0 2 Nu-3 30 30 9 7.3 0 0.6 Sigma-1 100 76

10 c 3 0 1 Sigma-2 (> 95%) Sigma-1 (< 5%) 60 44

a 140oc rather than 1800C bAbout 5% Sigma-1 occasionally obtained ¢200°C rather than 180°C

at these ratios

ZEOLITES, 1989, Vol 9, March 141


.(a)

i (b) 2

10.0

Figure 1 X-ray

1S.0 20.0 2S;.O 30 .0

Degrees Two Theta D

powder diffraction patterns (automatic slit divergence)'for samples of "as-made" Sigma-2 with different AI contents. (a) Si02/AI203 > 1000; (b) Si02/AI203 = 55

cm -1. The high-frequency region had a maximum absorbance at about 1100 cm -1 and a pronounced sideband at 1225 cm -a, previously ascribed to asym- metric Si-O stretching in a 5-membered ring.ll The deformations near to 500 cm-1 consisted of absorp- tions at 632 cm -1 (medium), 550 cm -1 (strong), 465 cm -1 (very strong), 444 cm -1 (weak), and 420 cm -I (weak). It is believed that this i.r. spectrum is unique to Sigma-2.

Calcination and t.g.a, studies The organic material in Sigma-2 proved difficult to

remove by simple calcination, particularly in the case of silica-only forms where black material resulted

Rgure 2 SEMs of Sigma-2 showing polyhedral crystals

even after prolonged calcination at 700°C. It was generally found that the more A1 present the more readily the organic could be burnt out of the samples. The findings are illustrated in Table 2. The use of 700°C calcination on a sample with SiO2/AI2Os = 48 reduced the carbon content to 0.3 wt% and gave a white product, which by XRD examination was still highly crystalline Sigma-2 (Figure 5).

T.g.a. (Figure 6) shed some light on these difficul- ties. With a sample of SIO2/A1203 = 50, little weight loss was observed below 450°C. The rate of loss only started to increase dramatically above 500°C, with the highest rate of loss and an associated exotherm being observed at 560°C. Once 800°C had been reached, the sample was held at that temperature until no further weight loss occurred. In all, a weight loss of 13.2 wt% was measured, with 7.5 wt% lost up to 800°C. The total loss corresponded very well with an organic content of 12.3 wt%, estimated from the elemental analysis for carbon.

Ion exchange properties No evidence of ion:exchange properties was found

for "as-made" or calcined forms of Sigma-2, even when Al-containing forms were used with 1 M HC1 solutions under reflux conditions. The alkali metal content (Na or K) was not changed. These results suggested that Sigma-2 did not possess pore openings or windows within its internal porosity that were large enough to permit exchange of Na + and K + ions under our exchange conditions.

Sorption studies Xenon (0.396 nm), water (0.265 nm), and n-hexane

(0.43 nm) were not adsorbed in experiments con- ducted at room temperature, indicating that rings of less than 8T-atoms formed windows that limited transport through the void volume of the material.

DISCUSSION

The use of low levels of alkali metal and aluminium in synthesis has, in combination with the 1-amine derivative of the cage organic molecule adamantane, led to the formation of the novel tectosilicate Sigma-2, which is characterized by its unique X-ray diffraction pattern. The present study suggests that this material has very small windows, smaller probably than 8- membered rings, since considerable difficulty was encountered in completely removing the organic species occluded within the pore system in synthesis. Once the pores had been freed of organic, water was still excluded from adsorption at ambient temperatures.

McCusker 12 has recendy determined the structure of Sigma-2 direcdy from synchotron X-ray powder diffraction data. The T-atom framework is shown in Figure 7, and the structure is composed of nonasil (4356) and larger eikosasil (51268) cages, both novel and previously unreported, even as theoretical units of structure. The structure designation SGT has been proposed by McCusker and Stewart and adopted by



Figure 3 Electron diffraction patterns and indexed idealized patterns for Sigma-2. (a) [100] zone-axis wi th Okl reflections such that k + I = 2n; (b) [110] zone-axis, at 45 ° to (a), wi th hhl reflections such that 2h + I = 4n



80

J 6O

i 40

20 --

' ' ' ' ' o ' o ' 'o ' ' ' 1600 1400 1 2 0 0 1 0 8 0 6 0 0 4 0 0

Wavenumber / cm -1

Rgure 4 I.r spectrum of "as-made" Sigma-2 as a Nujol mull

the Structure Commission of the International Zeolite Association. Is The large eikosasil cage can be envisaged as a tennis ball, with the twelve 5- membered rings making up the seam and with eight 6-rings formed by joining the T-atom species of the 5-rings through oxygens. McCusker also found that the 1-AN species, too large to be accommodated in the nonasil cage, was present in the large cage, and although disordered, one orientation that conformed to the 4 symmetry of the cage seemed to be preferred. The idealized unit cell composition reported by McCusker for the clathrasil form of Sigma-2 was [Si64Oi~8]'4 C]0Hx7N. Calculations here on the chemical analysis of "as-made" forms of Sigma-2, cor- rected for water content, give about one amine molecule to 15 T-atoms, indicating close agreement (to within 5%) for this composition and suggesting four large cages in every unit cell, Expressed in terms of T-atoms, the composition [T640128] '4 CIoHI7N would seem appropriate as a general formula for Sigma-2 materials made with adamantanamine.

Under the proposed classification of Liebau et al. 6 Sigma-2, with 17.9 T-atoms per 1000 A s, would belong to the class of porous tectosilicates. With its cage-like structure and tight windows controlling access to its intracrystalline void volume, the pure silica form would be described as a clathrasil, whereas the aluminosilicate form would be called a clathralite.

CONCLUSIONS

Sigma-2 is a novel tectosilicate, readily synthesized and recognized by its unique X-ray diffraction pattern. 1-Adamantanamine, the amine used to prod'uce

Table 2 Calcination studies on Sigma-2 samples

Calcination conditions Carbon content SiOz/AI203 (TempJ~C, time/h) Color (wt% C)

> 1500 500, 48 Black 8.2 870 500, 28; 700, 17 Black 1.8

91 500, 48; 700, 24 Dark grey 1.6 500, 48 Yel low 4.2

50 550, 16 Buff 2.7

500, 48 Yel low 4.6 48 500, 48, 700, 24 White 0.3

I0.0 1¢:.0 20.0 2g.O 30.0 Degrees Two T h e t a

Figure 5 X-ray powder diffraction pattern for Sigma-2 (SiO2/ AI2Oa = 48), calcined at 500°C for 48 h and then at 700°C for 24 h

800

(_;

o

600

o. E 400 p-

200 ~ l I ~1 , i I 20 40 60 80 1 O0 120 140

Time from 150°C / mlns

15

10uo :T

~n 5

Figure 6 Thermal analysis of "as-made" Sigma-2

it, is occluded at the level of four molecules per unit cell and is difficult to remove, particularly in clathrasil forms of Sigma-2. Once free of organic residues, Sigma-2 does not absorb even small molecules such as water, indicating very tight apertures within its intracrystalline void volume.

The cage-like amine, 1-adamantanamine, is well suited to a space-filling role in clathrasil and zeolite synthesis, to date having given rise to four different

Figure 7 T-atom framework of Sigma-2, showing the build-up of nonahedral units to produce eikosahedral cages


materials, all now of known structure. Since all have large cages and small 6- or 8-membered ring windows, it is less clear whether the structure-directing role of the amine may be described as more than that of favoring cage-type tectosilicates.

ACKNOWLEDGEMENTS

The author wishes to thank D.W. Johnson and P. Cowley for their help with sorption studies, M.D. Shannon for unit cell determination, G. von Bradsky for electron microscopy, M.B. Ward for technical assistance in synthesis and calcination studies, and ICI Chemicals and Polymers Limited for permission to publish.

REFERENCES 1 Liebau, F. Zeolites 1983, 3, 191


2 Gies, H. J. Inclusion Phenonena 1986, 4, 85 3 Gies, H., Liebau, F. and Gerke, H. Angew. Chem., Int. Ed.

Eng. 1982, 21,206 4 Marler, B., Dehnbostel, N., Eulert, H.-H., Gies, H. and Liebau,

F. J. Inclusion Phenomena 1986, 4, 339 5 Gies, H. Fortschr. Miner. 1985, 63, 74; J. Inclusion Phe-

nonena 1984, 2, 275 6 Liebau, F., Gies, H., Gunavardane, R.P. and Marler, B.

Zeofites 1986, 6, 373 7 Stewart, A., Johnson, D.W. and Shannon, M.D., 'Innovation

in zeolite materials science', in Studies in Surface Science and Catalysis, Voh 37 (Eds. P.J. Grobet et aL) Elsevier, Amsterdam, 1988, p. 57

8 Casci, J.L. and Whittam, T.V., Zeolltes: Synthesis, structure, technology and application, in Studies in Surface Science and Catalysis, Vol. 24 (Eds. B. Drzaj et aL) Elsevier, Amsterdam, 1985, p. 623

9 Stewart, A., in preparation 10 Stewart, A. and Hearmon, R.A., unpublished results 11 Jacobs, P.A., Beyer, H.K. and Valyon, J. Zeolites 1981, 1,161 12 McCusker, L.B., submitted for publication 13 Meier, W.M. and Olson, D.H., in Atlas of Zeolite Structure

Types, 2nd Revised Edn., Butterworths, London, 1987, p. 126


synthesis and characterization of the novel tectosilicate sigma-2

Documents