On the intermediates in zeolite Y synthesis

On the intermediates in zeolite Y synthesis B. Fahlke, P. Starke, V. Seefeld and W. Wieker

Central Institute of Inorganic Chemistry, Academy of Sciences of the GDR, DDR-1199 Berlin-Adlershof, GDR and K.-P. Wendlandt

Technical University 'Carl Schorlemmer' Leuna-Merseburg, DDR-4200 Merseburg, GDR (Received 22 February 1985; revised 18 June 1986) The liquid and solid intermediates in the synthesis of zeolite Y were studied by chemical and physical methods. The intermediates, obtained during aging and the first period of thermal treatment, are mixtures of a primary silica-rich gel and an aluminosilicate gel with a Si/AI ratio of 2.5. After 5 h thermal treatment until crystallization the X-ray amorphous solid product consists only of the aluminosilicate gel with 2.5 Si/Al. From the investigation of the liquid phases a dissolution-precipitation process can be proposed. Keywords: Synthesis; (Z) Y; intermediates; i.r.; catalysis

INTRODUCTION The synthesis and course of reaction of zeolite Y from silica sol and sodium aluminate solution have been frequently investigatedl-~. Particular attention has been given to the mechanism of formation, the influence of the starting materials and the processes occuring during aging. The aim of this work was to characterize by chemical and physical methods the X-ray amorphous intermediates obtained during zeolite Y synthesis, especially during thermal treatment. Information about the reaction ~xiechanism during zeolite Y synthesis can be obtained from knowledge of the chemical composition and the properties of the intermediates and their respective liquid phases.

do not react to form the yellow complex within 90 min. Use of the molybdate method for aluminosilicates is possible due to the immediate hydrolysis of Si-O-AI bonds in acid solution. The Si-O-Si bonds are not hydrolysed and react with the molybdic acid to give information about silicate building units of aluminosilicates. The solid phase was also characterized by i.r. spectroscopy. The i.r. measurements were made using a spectrophotometer DK 2A Beckman with KBr discs. The solubility in 0.2 N N a O H of some of the solid intermediates was investigated by stirring the mixtures at RT. One gram of solid was taken and enough sodium hydroxide solution to give a SiO2/Na20 ratio in reaction mixture of 1:1. Catalytic activity of the solid phases was tested by means of cracking: cumene to benzene and propene; as described in Ref. 14.

EXPERIMENTAL Intermediates and zeolite Y were produced as follows: Silica sol (30 wt% SiO2) was added slowly, with stirring, to a sodium aluminate solution in a 50 ml plastic beaker. The quantities of the reactants were chosen to give a molar ratio in the reaction mixture of 9SiO2"AI203"3Na20"I20H20. After addition of sol the reaction mixture was aged for 20 h at room temperature (RT). After aging the reaction mixture was divided and treated in stainless 50 ml autoclaves at 90°C without stirring. Autoclaves were opened after different reaction times. The solid and liquid phases were separated by centrifugation and the solid phases dried at 90°C. Photometric measurements showed that the liquid phase contained no colloidal particles. The solid and liquid phases were analysed g.ravimetrically and photometrically and charactenzed by the molybdate method 12"l" . The molybdate method is a kinetic method that gives information about the degree of condensation of the investigated silicate from the rate of formation of the yellow [B-dodecamolybdato silicic acid complex: H4SiO,I 412HzMoO4 ~ Ha[Si(OMo~O9).l]. It is possible to differentiate between molybdate active and molybdate inactive parts of the silicate. Molybdate inactive silicates are highly condensed and --

--

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RESULTS The reaction process is shown schematically in Figure 1 together with the designation of the samples taken after different synthesis times. Sample 00-00 is designated as the 'primary gel', the others as intermediates. Solid phases 00-00 to 20--48 are X-ray amorphous; sample 20-72 is crystalline and identical with zeolite Y. Figure 2 shows the Si and AI content in the aqueous phases. During aging at RT (293 K) the SiO2 content is constant and very low, whereas the AI content decreases from 0.77 to 0.35 mol !-l. At the start of temperature treatment, SiO2 content of the aqueous phases increases rapidly to 0.63 mol 1-l whereas the AI content falls sharply to 0.02 mol 1-J Figure 2 also shows that directly after the mixing of the reactants there is practically no SiO2 in the liquid phase. Therefore the precipitated primary gel must contain nearly all the silica present in the reaction mixture, resulting in a Si/AI ratio of between 12 and 15 Si/AI. Figure 3 shows that ratio in the gel falls to around 6 Si/A1; after the start of the temperature treatment the ratio falls even further and after 5 h reaches a value of 2.5-2.6. This ratio is also the ratio of the crystalline zeolite Y (sample 20--72). After

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silicate. The remaining SiO,, ill both the primary gel and intermediates could not be investigated, because it was so highly condensed or insoluble in 0.1 N HCI. Therefbre the molybdate reaction curves of the solid phases will not be discussed. Interpretation of the i.r. spectra is based on resuhs obtained for zeolite yl.~.l, and anaorphous, thermally decomposed zeolite Y. It was shown ~ that the position and width of the internal tetrahedra vibration bands vary only slightly between crystalline and antorphous zeolite Y. Therefore it is possible to exphfin i.r. results o1"amorplaous analogues using the known zeolite interpretations. Figure 5 shows the i.r. spectrum of the primary gel and intermediates compared with anaorphous silica gel, obtained fi'om silica sol with NaOH and zeolite Y (20-72). From the spectra it can be seen: • •

about 5 h thermal treatment the chemical composition of the solid and liquid phases remains constant; the liquid phase contains 1.78 moi Si per litre and 0.02 mol AI per litre. Figure 4 shows the molybdate reaction curves for the liquid phases together with calibration curves for particular silicate anions. It can be seen that directly after precipitation and at the end of the aging period silicate is present only as monomer. During temperature treatment the degree of condensation of silicate anions increases. All the SiO2 in all liquid phases is molybdate-active, this means that the liquid phases contain no highly condensed or colloidal SiO2. The solid phases contain only 5-20% molybdate-active SiO2 as low molecular

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the spectra of the primary gel and the silica gel are almost identical; the symmetric stretching vibration band at approximately 800 cm-~ disappears and shows the graduate dissolution of the SiO2 part in the primary gel and successive intermediates during aging and the first period of thermal treatment; the T-O stretching band shifts from 1085 to 1045 c m - l ; this implies a tetrahedral incorporation of • AI rata the gel f r a m e w o r k1'5. This• resuh was confirmed by ~-7 AI n.m.r, investigations; a new band at 710 cm -I appears with increasing intensity in the spectra of the sample 20-00 and all the following amorphous intermediates. The position of this band depends on the Si/AI ratio of aluminosilicates 15"16 and reveals a Si/A1 ratio of approximately 2.5 in the aluminosilicates of all intermediates•

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period and up to 10 h temperature treatment show a moderate cracking activity. Particularly interesting is the very high k-value observed for sample 20-48. Catalytic activity has increased by a factor o f - 10, but is not as high as that shown by crystalline zeolite Y. Reasons for this behaviour are being investigated.

DISCUSSION The primary gel and intermediates were further investigated by means of different solubility in 0.2 N NaOH. The SiO., part of the primary gel and the intermediates should dissolve much more quickly than the aluminosilicate gel in 0.2 N NaOH. The intermediates and the 0.2 N NaOH solution were stirred together for 1 or 6 h at RT and the solution and remaining solids analysed. Data are shown in Figure 6a and b. The highest and fastest dissolution of SiO2 is shown by primary gel and the intermediate 20-00. The Si/AI ratio in the residue of these two samples decreases especially quickly in the first hour of treatment with NaOH. Samples 20-24 and 20--48 show different solubility behaviour; dissolution proceeds slowly and a smaller amount of SiOz goes into solution. The Si/AI ratio of the residue remains constant during NaOH treatment. Sample 20-01 (only one hour thermal treatment) shows a solubility behaviour between the two extremes. Results of the tests for catalytic activity are shown in Figure 7; rate constants in dependence from the time of synthesis. The samples taken during the aging

Characterization of the X-ray amorphous intermediates in zeolite Y synthesis using various methods taken together with the investigation of the corresponding liquid phases shows the following. Intermediates are a mixture of SiO2-rich primary gel and a sodium aluminosilicate phase, which has a Si/A1 ratio of 2.5. The relative amount of these two phases varies. Immediately after mixing the reactants and during aging time the solid phase contains predominantly SiO2 gel. These findings agree with those of other authors =--6. In the first hours of the reaction, especially during aging time, the SiO2-rich primary gel goes into solution as low molecular silicate anion. The presence of low molecular silicates in the liquid phase during zeolite Y synthesis was supposed from Raman spectroscopic measurements by Roozeboom 7 and confirmed by the molybdate m e t h o d mentioned above. T h e dissoluted low molecular silicates react with the aluminate anions already present in the solution and probably form aluminosilicate anions. Zhdanov 2's and Roozeboom 7 supposed the formation of soluble aluminosilicate species during zeolite Y synthesis. The results of

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products the composition, i.r. spectra and solubility in 0.2 N NaOH remain unchanged. These results suggest a complete dissolution of the SiO2-rich primary gel. The total amount of primary gel is converted into a solid aluminosilicate phase with 2.5 Si/AI via the liquid phase. Probably during the dissolution-precipitation process the liquid phase is supersaturated in the aluminosilicate nuclei. Therefore, the precipitated aluminosilicate particles are very small and undetectable by X-ray diffraction. This suggestion corresponds with findings of Roozeboom et al. 7. They revealed that the characteristic XRD peaks appear when zeolite crystallites are - 5 0 0 /~ in size. Conversion of the X-ray amorphous aluminosilicate particles into crystals are being investigated. Rearrangement processes probably take place, similar to those reported by Hino 9 for the conversion of amorphous phases into crystals during zeolite Y synthesis derived from tetraethylorthosilicate.

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CONCLUSIONS When the results from investigation of solid phases are taken together with those from liquid phases the following reaction steps, during zeolite Y synthesis derived from silica sol, can be proposed: •

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Catalytic activity of the intermediates and zeolite Y in cumene cracking. Key as in Figure 7

• Ueda II provided the evidence of the presence of soluble aluminosilicate species during zeolite Y crystallization from clear solutions by n.m.r. However, it was impossible to confirm the presence of aluminosilicate species in the liquid phase of the investigated zeolite Y synthesis due to their very low concentration. When the SiO2 concentration in solution reaches 0.02 mol 1-I the aluminosilicates precipitate from solution as a phase with Si/Ai ratio of 2.5, containing low m o l e c u l a r silicate b u i l d i n g units. T h i s dissolution-precipitation process continues during aging and the first hours of temperature treatment, until practically all the aluminate anions are taken into the gel phase. T h e n the residual primary SiO2-rich .gel goes further into solution. The SiO2 concentrauon and, therefore, the silicate anion condensation degree in the liquid phase increase. After approximately 5 h t e m p e r a t u r e treatment the dissolution-precipitation process is complete. From this time until crystallization occurs the composition and silicate anion c.ondensation degree of the liquid phase remain constant. Also, in the intermediate

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immediately after mixing the reactants a predominantly SiO2 gel containing primary gel appears; during aging time and the first hours of thermal treatment the primary gel goes into solution as low molecular silicate anion. These silicate anions react with aluminate anions already present in the solution; after reaching a certain concentration the aluminosilicate species precipitate from solution as a gel with a Si/Ai ratio of 2.5, containing low molecular building units; this dissolution-precipitation process continues until practically all the aluminate anions are taken into the gel phase; the dissolution-precipitation process is completed after ~ 5 h thermal treatment. From this time until crystallization occurs the composition and the behaviour of the solid and liquid phases remain unchanged; it can be concluded that the conversion of the primary SiO2-rich gel into the solid intermediates with 2.5 Si/AI occurs as dissolution-precipitation process via liquid phase. The slightly ordered structure of the X-ray amorphous solid intermediates with 2.5 Si/AI probably undergoes a rearrangement to form a crystalline structure.

ACKNOWLEDGEMENT We would like to acknowledge Dr. G. Engelhardt; Central Institute of Physical Chemistry; Academy of Sciences of the GDR, for 27A1 n.m.r, spectroscopy.

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Intermediates in zeolite Y synthesis: B. Fahlke et al. 3 Zhdanov, S.P. 'Proc. III Int. Conf. Molecular Sieves', 1973, p. 25 4 Fahlke, B., Wieker, W., F~rtig, H., Roscher, W. and Seidel, R. Z. Anorg. AIIg. Chem. 1978, 439, 95 5 Polak, F. and Cichocki, A. Adv. Chem. Ser. 1973, 121,209 6 Polak, F. and Stobiecka, E. Bull. Acad. Polonaise Sci. Ser. Sci. Chim. 1968, 26, 899 7 Roozeboom, F., Robson, E.E. and Chan, S. Zeolites 1983, 3, 321 8 Kacirek, H. and Lechert, H. J. Phys. Chem. 1975, 79, 1589 9 Hino, R., Matuura, R. and Toki, K. Bull. Chem. Soc. Jpn. 1983, 56, 3715

10 Hino, R., Aol, Ho and Toki, K. Bull. Chem. Soc. Jpn. 1984, 57, 317 11 Ueda, So, Kageyama, N. and Koizumi, M. 'Proc. VI Int. Conf. Molecular Sieves' 1983, p. 905 12 Thilo, E., Wieker, W. and Stade, H. Z. Anorg. AIIg. Chem. 1965, 340, 261 13 Fahlke, B. and Wieker, W. Zeolites 1983, 3, 195 14 Bremer, H., Jank, M., Fahlke, B., Starke, P. and Wendlandt, K.-P. Z. Anorg. AIIg. Chem. 1983, 500, 51 15 Flanigen, E.Mo, Khatami, H. and Szymanski, H.A.Adv. Chem. Ser. 1971, 101,201 16 Lohse, U. Dissertation B Berlin, 1981