Synthesis and Characterisation of Crystalline Aluminosilicate Sigma-1.

P .J. Grobet et al. (Editors) / Innovation in Zeolite Materials Science Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

57

SYNTHESIS AND CHARACTERISATION OF CRYSTALLINE ALUMINOSILICATE SIGMA-I.

A. STEWARTl, D.W. JOHNSON2 AND M.D. SHANNON2 Imperial Chemical Industries PLC, Chemicals and Polymers Group, R&T Dept., lp.O.Box 90, Wilton, Middlesbrough, Cleveland TS6 8JE, and 2p.0.Box 11, The Heath, Runcorn, Cheshire WA7 4QE, England.

ABSTRACT A new crystalline aluminosilicate zeolite, Sigma-I, with a structure similar to the pure silica clathrasil, deca-dodecasil 3R, has been made with l-adamantanamine and found to have small-port sorption characteristics with a pore constraint of about 0.43 nm. Evidence is presented that Al is in framework T-atom positions, giving rise to cation-exchange properties and acid catalysis. Shape selectivity in the product distribution from methanol conversion is considered in the light of the known structure of deca-dodecasil 3R. INTRODUCTION Amine~

have been used successfully as directing agents in the synthesis of

tectosilicates (ref. 1) and the family of molecular sieves based on crystalline aluminophosphate frameworks (refs. 1,2). The amine, l-adamantanamine (AN), has been used by Gies in the synthesis of the clathrasils, dodecasil lH (ref. 3) and deca-dodecasil 3R (refs. 4,5). These materials were prepared in the absence of any alkali metal or alkaline earth cations. In this paper we report the synthesis and characterisation of a new crystalline aluminosilicate zeolite, Sigma-l ([-1) prepared using AN and alkali metal hydroxide at low levels of OH- relative to silica in the reaction mixtures. We also describe the properties of this material when it is activated to its H form. Particular attention is paid to establishing that aluminium is present in the framework of this tectosilicate. Acid catalysis is demonstrated by conversion of methanol to hydrocarbon products. EXPERIMENTAL Syntheses were carried out under autogeneous conditions at l80·C and 500 rpm in stirred stainless steel autoclaves, using colloidal silica, "Syton X30" (Monsanto Chemical Co.) or "Nalfloc 1034A" (Nalfloc Ltd.), sodium hydroxide and sodium aluminate (BDH Chemicals Ltd.), and AN (Aldrich Chemical Co. Ltd.). Products were removed, filtered, washed and dried at 110·C.

58

Samples of HL-1 were prepared by calcination of dried "as-made" products at 500°C for 48 hours, followed by ion exchange using 1M HC1 solution (10 cm3 per g zeolite) for Z hours at room temperature. The ion exchange was repeated after filtration. The Na-form was made by exchange of the H-form using 1M NaC1 solution (Z5 cm3 per g zeolite, 60°C, and repeated). Samples of the NH4-form were made from the Na-form by contacting it with 1M NH4C1 solution (10 cm3 per g zeolite, Z hours at room temperature, repeated). Samples were examined by scanning electron microscopy (JEOL 100CX microscope), x-ray diffraction (Philips APD 1700 automated x.r.d

syste~

using

Cu Ka radiation), electron diffraction (Philips EM400T microscope) and infrared spectroscopy (Perkin Elmer 580 spectrometer). Z7A1 MASNMR spectra were obtained with a JEOL FX-ZOO QS spectrometer, equipped with a Chemagnetics double-bearing MAS probe. Thermogravimetric analysis (tga) and differential thermal analysis (dta) were done simultaneously on a Stanton Redcroft STA 781 instrument. Crystal density was determined using l,Z-di-n-buty1phthalate (kinetic diameter> 1.0 nm), previously dried with molecular sieve 3A. Samples of zeolite (H-form) were fully dehydrated on a vacuum line before transfer to a dry box for addition to the organic compound. The adsorption of ammonia was studied using a zeolite powder sample eH-form) in a diffuse reflectance cell using a Nicolet 170SX F.T.I.R. spectrophotometer. Nitrogen isotherms were determined volumetrically using Micromeritics Digisorb Z500 equipment. Uptake of a range of other gases was determined gravimetrically using a Sartorius Model 4433 vacuum microbalance. Full adsorption isotherms were measured up to a relative pressure of 0.9 and the zeolitic micropore volume was calculated using the alpha-plot method (ref. 6). Catalytic runs were carried out in a flow microreactor using pure zeolite in the form of aggregates within a size range of 500 to 1000 microns. Methanol was vaporised, mixed with nitrogen diluent and passed over the catalyst. Product streams were analysed using on-line gas chromatography. RESULTS Synthesis and Identification of Products In the NaZO, AN system at 180°C, we have found that pure r-1 was made most readily at a molar ratio of SiOZ/AlZ03 of about 60 in the reaction mixture when low ratios of free OH-/SiOZ were employed. (The ratio, free OH-/SiOZ, may be calculated by the method of Ro11mann and Va1yoscik (ref. 7) and uses the Na+ level remaining once strong acid and aluminate (A10Z-) anions are satisfied.) Table 1 shows the effect of varying this ratio on the products formed at SiOZ/A1Z03 = 60. It is clear that, as far as zeoli tic compounds were concerned

59

TABLE 1 Effect of Varying Free OH-/SiOZ Ratio in Reaction Mixtures on Products Formed. (x NaZO: ZO AN: 1 AlZ03: 60 SiOZ: Z400 HZO; 180 o e, 500 rpm) Prep No.

x

1 Z 3 4

3 5 10 17

Free OHSiOZ 0.067 0.133 0.300 0.533

Time (d)

Final pH

Products Obtained

6 5 5.5 6

11.72 12.10 1Z.03 1Z.01

Sigma-1 Sigma-1 ZSM-5,CX-cristoba1ite mordenite, CX-quartz,CX-cristobalite

mordenite was favoured by high free

O~/SiOZ

ratios, while at intermediate ones

ZSM-5 type product resulted. At these high ratios crystalline silicas such as a-quartz and a-cristobalite were found as well. Using compositions otherwise similar to those of Prep.l (Table 1) but omitting AN, we found that only amorphous material resulted with no hint of r-1 after ZO days. Table Z illustrates the effect of varying AlZ03/SiOZ ratio on a series of experiments conducted with otherwise similar relative compositional ratios; with little or no AI, a novel crystalline silicate, designated r-2 was formed (ref. 8), while increased amounts results in r-l and eventually Nu-3 (ref. 8). In this series the trend was that the final pH increased with the amount of AI. r-Z was most readily prepared in a highly siliceous form, although it can be made with Si02/AlZ03 ratios in the region of 50 (ref. 8). Increasing the temperature from 180 to

zoooe

with x = 3 and y = 1 resulted in the formation of

r-Z, containing only about 5% r-1. When the temperature was lowered to 160°C, crystallisation proceeded more slowly but r-1 was still the product. As crystallisation in Prep.1 (Tables 1 and 2) went toward completion, pH increased and reached a steady value. The product was analysed and found to contain 0.3% Na, 1.21% AI, 34.7% Si, 7.4% C and 0.9% N (i.e. mole ratio SiOZ/ AIZ03 = 55). ilL-I had the x.r.d. pattern shown in Fig. l(a), and, when examined by SEM, was found to be in the form of rounded tablets, about Z.5 microns in diameter, aggregates of smaller zeolite crystals (Fig. 2). Electron TABLE Z Effect of Changing A1203/SiOZ Ratio in Reaction Mixtures on Products formed. (x NaZO: ZO AN: y AlZ03: 60 SiOZ: Z400 HZO; 180 o e, 500 rpm, x ~ 3) Prep No. 5 6 1 7 8

x

y

3.00 Z.97 3.00 3.21 3.00

0.00 0.61 1.00 1.50 Z.OO

Free OHSiOZ 0.100 0.079 0.067 0.054 0.006

Time (d)

Final pH

Products Obtained (Approx. % ages)

6 5 6 8 9

11. 55 11. 73 11. 7Z 11.94 11.88

Sigma-Z(100%) Sigma-2(85%), Sigma-l(15%) Sigma-l(100%) Nu-3(50%), Sigma-1(SO%) Nu-3(lOO%)

60

(a)

5

10

15 20 25 Degrees 2-theta --+

Fig. 1 (a) Experimental x.r.d. pattern for H Sigma-l (b) Simulated x.r.d. pattern for deca-dodecasil 3R (Both for automatic slit divergence).

30

Fig. 2 Scanning Electron Micrograph of Sigma-l (Scale 0.75 cm = 1 micron).

diffraction indicated a rhombohedral unit cell with approximate lattice parameters a = 1.37 nm, and c = 4.08 nm. Comparison of the x.r.d. pattern and cell parameters with those of deca-dodecasil 3R (refs. 4,5) led us to believe that the two materials had essentially similar frameworks. To check this the atom positions given by Gies (ref. 5) for deca-dodecasil 3R were entered into the POWD.12 programme (ref. 9) to simulate the x-ray powder diffraction pattern (Fig~

l(b)). This showed very good agreement with our experimental data.

Characterisation Data from thermal analysis of as-made L-l (Prep.l, Table 1) are given in Fig. 3. Weight loss below 250°C was assumed to be due to water loss, while that between 250 and 800°C to be predominantly due to decomposition and combustion products from the organic content of the as-made sample. The rate of loss started to increase dramatically at 420°C, two stages being observed with exotherms in the dta curve at 472 and 610°C. Between 250 and 800°C, weight losses accounted for some 10% of the original sample weight, a value relatively close to the organic content of 9.4%, estimated from elemental analysis, assuming that the organic was present as AN in the as-made product. This was confirmed by 13C MASNMR, which showed that the aminewas in fact in a protonated form occluded

within the zeolite (ref. 10). X.r.d. analysis of a sample

calcined at 700°C in a muffle furnace confirmed that it was still highly crystalline

I-i.

However, it was not necessary to use such extremely high calcination temperatures to remove the organic and this was possible at 500°C, although

61 100

11 0

i

x

OJ

i

Ul Ul

0 ,....,

....,

~

3:

""

0

"0

c

OJ

200

400

Temperature/DC

600

i

80

<::

.S 60 Ul Ul .c<

8

~

(lj

40

L

E--<

"" 20 1200

800

800

Wavenumbers/cm- 1

~

400 -

Fig. 4. I.R. Spectrum of H Sigma-l (as a Nujol Mull.)

Fig. 3. Thermal Analysis of "As-made" Sigma-l

times up to 2 or 3 days were required to get any remaining residues to low levels «0.3% C), a finding consistent with the difficulty encountered in the tga experiment. Once calcined, the Na content was reduced by applying ionexchange procedures. Two exchanges with Hel solution brought the Na content to 0.008 wt% with the Si02/A1203 mole ratio little changed at 54. The sorption properties of HL-l were probed using a range of small molecules. Examination of the results in Table 3 revealed a sharp cut-off for molecules with kinetic diameters above 0.4 nm. This would put the material into the small-port category of zeolites with 8-T atom windows. Interestingly, under the conditions employed the linear alkane, n-butane, was not adsorbed, although at higher temperatures this may be possible. (Linear alkanes were produced in methanol conversion reported below.) The crystal density was measured as 1.74 g cm- 3, equivalent to 117 T-atoms TABLE 3 Adsorption of Various Sorbates in HL-l (Single point Method unless otherwise stated) Sorbate water nitrogen a) methanol xenon b) n-butane i-butane benzene

Kinetic Diameter (nm) 0.265 0.364 0.380 0.396 0.430 0.500 0.585

Temp Partial Pressure p/Po (K) 293 77

295 293 273 193 293

0.5 range 0.3 range 0.5 0.5 range

Uptake (wt %)

Apparent Voidare Filled (cm 3 g- )

10.5 9.8 9.2 20.9 0.1 0.3 0.2

0.105 0.121 0.116 0.105 0.002 0.003 0.002

a) a-plot method b) Extrapolation of isotherm of Langmuir type; density = 1.99 g cm- 3, obtained by comparison of uptakes of Xe and N2 on a range of zeolites (ref. 11).

62 per unit cell. This was in good agreement with the theoretical number of 120 derived for the structure of deca-dodecasil 3R (ref. 5). HL-l displayed an i.r. spectrum typical of a zeolite (Fig. 4), with intense T-O stretching modes at about 1100 cm- l and bending modes around 500 cm- l• The high-frequency region was typical of a high-silica zeolite (ref. 12) with maximum absorbance at 1070-1120 cm- l and a pronounced sideband at 1220 cm- l, which has been previously ascribed to asymmetric Si-O stretching in a 5membered ring (ref. 13). The deformations near to 500 cm- l consisted of absorptions at 614 cm- l (weak), a doublet at 528 and 517 cm- l (medium) and 462 cm- 1 (medium-strong). Although individually these peaks are found in other zeolites, the set is unique to r-1. Location of Aluminium [-1 seemed to have the framework topology of deca-dodecasil 3R. The question of whether Al was present as a framework T-atom was answered by 27Al MASNMR. This spectrum showed that the Al in an as-made sample

(Si02/A1203~50)

was

tetrahedrally coordinated with no evidence for octahedral AI. More evidence for Al as a framework T-atom came from ion-exchange studies, conducted on an aliquot of the same batch which had been calcined and acid exchanged as above (0.01 wt% Na). It was possible to make the Na-form (0.7 wt% Na) by a double exchange with NaCl solution, and then to convert that to an NH4-form (0.01 wt% Na) by using NH4Cl solution. Thus, cation exchange properties were demonstrated for [-1. Acidity of the H-form was shown in NH3 chemisorption experiments, monitored by i.r.spectroscopy. A powder sample of HI-l, pumped to about 350°C, exhibited two absorptions at 3740 and 3604 cm- l, usually recognised as originating from surface silanols and internal Bronsted acid sites respectively (see for example, ref. 14). Both bands disappeared on adsorption of NH3 at ambient temperature, indicating that all the hydroxyls were accessible to the base. Upon pumping to lOO°C the 3740 cm- l band returned, confirming the essentially non-acidic nature of the silanols. The 3604 cm- l band was due to hydroxyls of high acid strength since it was not recovered until the sample was pumped at temperatures of above 400°C. In methanol conversion studies the activity of H[-l was found to decay rapidly with time-on-stream, as shown in Table 4. However there was clear evidence of a high degree of shape-selectivity with the lower alkenes predominating and no sign of aromatics in the product stream. Ethene and propene were principal products, together accounting for >60% of the carbon found in the product stream. Such selectivity was in line with findings on other small-port zeolites, e.g., FU-l (ref. 15) and Nu-3 (ref. 16).

63 TABLE 4 Distribution of Hydrocarbons in the Product Stream from Methanol Conversion over HI-l at 400°C (Molar Ratio N2/MeOH in feed = 2, WHSVMeOH 1.1 h- 1). Time on Stream (mins)

15

45

75

Conversion of MeOH to Hydrocarbons (%)

89

28

19

Aliphatic Products (as % of Total Carbon in Hydrocarbon Products) Methane Ethane Ethene Propane Propene Butene C5+ a1iphatics

3.1 2.6 18.8 4.5 45.6 13.4 12.0

3.1 3.3 19.4 0 54.5 14.9 4.8

3.3 2.7 19.5 0 59.8 14.7 0

77.8

88.8

94.0

Ethene + Propene + Butene

DISCUSSION I-I was made with 1-adamantanamine, the same amine that was used by Gies to prepare deca-dodecasil 3R (refs. 4,5).

Unlike the case of the clathrasil which

could not be prepared at temperatures of greater than 170°C (ref. 4), I-I was readily prepared in our systems (with alkali metal involved) at 180°C (and above). At higher temperatures Gies found that dodecasil IH was produced instead of deca-dodecasi1 3R (ref. 4), whereas in our system we have found that I-lor the novel material, I-2, was formed, depending on the alumina content of the reaction mixture, I-2 being favoured at low AI. Subtle differences in the synthesis were brought about by the alkali metal and by the incorporation of alumina. The characteristics observed for zeolite I-I (x.r.d., tga, pore constraint, and product distribution from methanol conversion) are consistent with it having the same framework topology as deca-dodecasil 3R, but with its behaviour modified by the presence of framework AI. Unlike deca-dodecasil 3R which has been found to show no weight losses below 500°C (ref. 4), I-I loses weight from about 420°C, possibly due to the effect of its aluminium. There can be little doubt from the evidence presented here that AI's are present as framework Tatoms and that these, together with the porosity generated upon calcination, give rise to true zeoli tic properties (sorption, ion-exchange, acidity when converted to H-form), albeit with a pore constraint of about 0.43 nm. The sorption properties of I-I can be understood once the cage sizes and window shapes have been calculated using the structural information obtained for decadodecasil 3R (ref. 5). The equilibrium window size for the elliptical entrance into the large 19-hedron cage is calculated ·by us to have major and minor axes

64 of 0.45 and 0.36 nm respectively. Small molecules «0.4 nm kinetic diameter) readily adsorb. Since Xe adsorbs into I-I, some flexibility of the framework at the window must exist. For comparison, Nu-3, which has the framework topology of Levyne and which also adsorbs Xe (ref. 11), has an elliptical window, 0.48 x 0.32 nm. In both these cases the areas of the windows are the same as a circular window of 0.40 nm diameter which would permit Xe to pass. The calculated microporosity of deca-dodecasil 3R is 0.175 cm3g-1 assuming that the 19-hedrons are the only cages accessible to sorbates. Our experimental figures for I-I are however some 30% lower, possibly due to inefficient packing or incomplete calcination of the samples examined. Currently the series of topologically-different materials synthesised pure with 1-adamantanamine present as guest molecule numbers at least four, namely, dodecasil IH, deca-dodecasil 3R (L-l), Nu-3 and [-2, thereby delQonstrating the complex interplay between synthesis conditions and the structure which results. All would appear to be cage structures with small windows, showing the ability of the organic amine to stabilise such cages. ACKNOWLEDGEMENTS We wish to thank B.W. Cook, N. Poole, Dr. R.A. Hearmon, Dr. S.V. Norval, G.S. Anderson and H. Ogburn for diffraction work, spectroscopy and microscopy, P. Whitney, R.M. Daniels, and M.B. Ward for technical assistance, and ICI PLC for permission to publish. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

B.M. Lok, T.R. Cannan and C.A. Messina, Zeolites,3 (1983) 282-291. S.T. Wilson, B.M. Lok, C.A. Messina, and E.M. Flanigen, in D. Olson and A. Bisio (Eds.), Proc. 6th Int. Conf. on Zeolites, Butterworths, Guildford, 1984, pp. 97-109. H. Gies, J. Inclusion Phenomena,4 (1986) 85-91. H. Gies, Fortschr. Miner.,63 (1985) 74; J. Inclusion Phenomena,2 (1984) 275-278. H. Gies, Z. Krist.,17S (1986) 93-104. S.J. Gregg and K.S.W. Sing, Adsorption, Surface Area and Catalysis, 2nd Edition, Academic Press, 1982, p. 98. L.D. Rollmann and E.W. Valyocsik, Zeolites,S (1985) 123-125. A. Stewart, to be published. D.K. Smith and M. Holomany, Fortran IV Programme for Calculating X-ray Powder Diffraction Patterns, Pennsylvania State University, 1986. R.A. Hearmon and A. Stewart, to be published. D.W. Johnson, to be published. J.C. Jansen, F.J. van der Gaag and H. van Bekkum, Zeolites,4 (1984) 369-372. P.A. Jacobs, H.K. Beyer and J. Valyon, Zeolites,l (1981) 161-168. J. Dewing, F. Pierce and A. Stewart, in B. Imelik et al. (Eds.), Studies of Surface Science and Catalysis, Vol.5, Catalysis by Zeolites, Elsevier, Amsterdam, 1980, pp. 39-46. M.S. Spencer and T.V. Whittam, U.K. Patent 1,563,345 (1980). J.L. Casci and T.V. Whittam, in B. Drzaj et al. (Eds.), Studies of Surface Science and Catalysis, Vol.24, Zeolites - Synthesis, Structure, Technology, and Application, Elsevier, Amsterdam, 1984, pp. 623-630.