J. Chem. SOC., Faraday Trans. I, 1986, 82, 2645-2649 Distribution of Silicon-to-Aluminium Ratios in Zeolite ZSM-5 Jia-Ching Lin and Kuei-Jung Chao* Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan, Republic of China The distribution of silicon-to-aluminium atomic ratios in ZSM-5 crystals has been measured. Particles of ZSM-5 with different sizes may have uniform chemical composition across the particle. In the same batch product, the Si/A1 ratio of each crystal can be different. A number of studies concerning the distribution of aluminium concentration in zeolite ZSM-5 have appeared recently.1-6 By Auger electron spectroscopy (AES) the secondary- ion mass spectroscopy (SIMS), Suib et a1.l and Dwyer et al., obtained a homogeneous aluminium distribution in polycrystalline and in single crystalline (25 pm) materials.Moreover, Si enrichment on the surface of the ZSM-5 cluster and A1 enrichment on the surface of 50-200pm large crystals were observed, respectively, by Hughes et aL3 by using X-ray photoelectron spectroscopy (XPS), and by von Ballmoos and Meier4 from an electron microprobe analysis (EPMA). On the other hand, Derouane et ~ 1 . ~ reported that for large crystals (5-8 pm), surface and bulk Si/A1 ratios were similar; for smaller crystals (< 1 pm), an Al-enriched surface was present. Lyman et a1.6 deduced, by means of transmission electron microscopy (TEM), that particles of ZSM-5 with different Si/Al ratios may have different A1 profiles across the particle. In this paper we report a detailed investigation of the A1 content in ZSM-5 crystals obtained by various synthetic routes, applying surface and bulk characterization analyses to several isolated phases in reaction mixtures.Our results may explain the seeming disparity in the results mentioned above. Experimental Zeolite ZSM-5 of two different sizes (ca. 50-100 pm and ca. 3-5 pm) was prepared by hydrothermal crystallization of aluminosilicate hydrogels in the presence of tetrapropyl- ammonium cations (TPA). ZSM-5 of ca. 3-5pm was synthesized from aluminium sulphate (Merck), waterglass solution of 21.6% SiO, and 8.1 % Na,O, sulphuric acid and tetrapropylammonium bromide (TPABr, WiiKo Chemicals), with Si0,/A120, z 35-140 as previously reported7 at pH 11 and 167 "C for 8-12 days. A high alkalinity was employed for the synthesis of large ZSM-5 crystals.The mixture of sodium aluminate, Ludox HS-40 colloidal silica (Du Pont), sodium hydroxide with TPABr in SiO, : Al,O, : TPABr : NaOH : H 2 0 = 10 : 1 : 30 : 30 : 2000 was reacted at 187 "C for 12 days. The variation of the Si/Al ratio in ZSM-5 was measured by EPMA and AES across an individual zeolite crystal and between crystals in the crystalline products. The bulk compositions of the reaction solution and solid product were analysed by induced- coupling plasma atomic spectroscopy (ICP). Electron microprobe scans were performed on a Joel superprobe 733 instrument at 25 KV, 0.02 pA sample current with a beam diameter of ca. 2-5 pm. Three measurements were taken at each point with an average counting time of 5 s. For scanning Auger electron spectroscopy a Perkin-Elmer PHI 590 26452646 70 60 0 .* w E 0 50,- 7 LO- *g Y > m Distribution of %/A1 in Zeolite ZSM-5 - - a a a a a I I I 1 I I 10 20 30 40 50 distance/pm Fig.1. The profile of %/A1 ratio across a section of a ZSM-5 crystal [plate 1 (b)]. 1 I I I I I 10 20 30 LO 50 distance/pm Fig. 2. The profiles of Si/A1 ratio across a section of two twinned crystals: A, along --- -+ of plate 2(a); A, along- of plate 2(a); 0, along --- -+ of plate 2(b); a, a l o n g - - - + of plate 2(b). AM system was used. Quantitative evaluation of the surface composition from AES data was achieved by recording the peak-to-peak amplitude of respective elements at 3.0 keV and 1.0 ,uA and by calculation using the corresponding sensitivity factors.* A small current from the neutralizer was applied in order to reduce the sample charging.Depth profiles were obtained by repeatedly monitoring the appropriate Auger transitions with simultaneous 3.8 keV Ar+ ion sputtering at an Ar partial pressure of ca. 5 x Torr.? The bulk %/A1 ratio of the sample was examined by ICP emission spectrometry. The solid sample was dissolved in acids and analysed by Plasmakon S35. t 1 Torr = 101 325/760 Pa.J-C. Lin and K-J. Chao 2647 Table 1. Compositional variations across large ZSM-5 crystals crystal size/,um point analysis average Si/A1 ratio 70 x 43 x 22 58 x 40 x 24 93 x 39 x 26 104 x 52 x 26 105 x 30 x 15 1 0 0 x 3 7 ~ 3 3 ~ 50 x 25 x 16b 80 x 41 x 21b 10 10 20 15 10 10 5 5 33f2 40f3 49&2 46f 1 30+ 1 23+2 32+2 45+3 a Twinned crystal of plate 2(a).Twinned crystal of plate 2(b). Table 2. The Si/A1 ratios of the reaction mixtures in ZSM-5 formationa initial reaction solid ZSM-5 solution mixture product cry s t a1 residue 18 19 CU. 18-20 - 35 26 ca. 24-46 - 70 31 CU. 30-120 - 23 ca. 20-30 3.5 x 103 3 5 w 25 - 7.7 x 102 3509 a Bulk Si/Al ratios of solid products and solution residues were determined by ICP. The Si/Al ratio of ZSM-5 crystal was determined by electron microprobe scan of 5-10 crystals. During the growth of large ZSM-5 crystals the crystalline products were contaminated with gel, as observed by SEM. The products were washed with very dilute alkaline solution and de-ionized water. Results and Discussion The scanning electron micrographs of typical single, twinned and powdery crystals are shown in plates 1 , 2 and 3, respectively.Single crystal and powder X-ray diffractions were used to identify the ZSM-5 str~cture.~ The typical A1 concentration profiles of single and twinned crystals are given in fig. 1 and 2. The dispersion of the Si/Al ratio across an individual single crystal is homogeneous. Two types of the twinned crystals were obtained as shown in plate 2(a) and (b). A uniform Si/Al ratio in the crystal of plate 2(a) is observed. The crystal shown in plate 2(b) is composed of two parts with different average Si/Al ratios as shown in table 1 and fig. 2. The variation of aluminium in ZSM-5 crystals produced from the same reaction batch is shown in table 1 . The inhomogeneity of aluminium concentration in ZSM-5 lies between the crystals rather than within a single crystal.Thus, ZSM-5 crystals with different Si/Al ratios can be formed in the same reaction mixture. The crystallization of zeolite from hydrogel includes nucleation and crystal growth. The rate of nucleation is strongly affected by hydroxide ions and by aluminium concentration in the reaction solution. The presence of hydroxide ions accelerates the dissolution of aluminosilicateJ. Chern. SOC., Faraday Trans. I , Vol. 82, part 9 Plates 1 and 2 Plate 1. Scanning electron micrographs of typical large ZSM-5 crystals: (a) crystal for X-ray diffraction; (b) crystal for Si/Al ratio measurement. Plate 2. Scanning electron micrographs of typical twinned crystals. J-C. Lin and K-J. Chao (Facing p . 2648)J .Chem. SOC., Faraday Trans. 1, VoE. 82, part 9 Plate 3. Scanning electron micrograph of typical ZSM-5(35) Plate 3 J-C. Lin and K-J. Chao2648 120- 90- 0 .- c., 2 o 60- 4 0 'S c, m d k m 30 Distribution of %/A1 in Zeolite ZSM-5 - I I I I I I I 1 1 I 2 L 6 8 10 analysis point Fig. 3. The profiles of Si/A1 ratios of small crystals in 0, ZSM-5(18); A, ZSM-5(35); H, ZSM-5(70); one analysis point per crystal. and polysilicate hydrogels. The dissolved silicate and aluminate ions can undergo a polymerization process and regroup around the hydrated cations to form the nuclei of the ordered zeolite. Since the aluminate in solution can form Al(OH),,, which consumes OH- ions in the solution, the quantity of the available OH- ions for depolymerization of gel probably depends on the aluminium concentration in the reaction mixture. Thus, the rate of the crystallization of zeolite ZSM-5 in an Al-rich environment will be slower than that in an Si-rich environment.Large ZSM-5 crystals of Si/Al ratio of ca. 23-49 were formed with analcime (Si/Al = ca. 3-5) and a trace of quartz. The Si/Al ratio of 23 corresponds to the average number of A1 atoms per unit cell being 4 and A1 atoms per channel intersection being 1 in ZSM-5. The negative charge of the aluminate anion resulting from the replacement of Si atoms by A1 atoms in the framework is neutralized by the positive charge of the TPA template ion, The bulk Si/Al ratio of the solid product of the Si02:A120,: TPA: NaOH: H,O = 10: 1 : 30: 30: 2000 batch is 7.7. The aluminium content of the crystalline ZSM-5 is lower than that of the reaction mixture and of the solid product.Zeolite ZSM-5 was produced from the reaction mixture with the Si/Al ratio varying from 18 to 70; it is the only crystalline product in which the bulk Si/A1 ratio is ca. 19-3 1, much lower than those in the reaction mixtures. Crystals of low Si/A1 ratio may also form in an Si-rich environment and these tend to withdraw A1 species from the liquid during growth. Therefore, the aluminium concentration varies during autoclave treatment of the reaction mixture, especially in reaction mixtures with a high Si/Al ratio, such as the %/A1 = 70 batch, in which the distribution of %/A1 ratios between ZSM-5 crystals is more dispersed, as shown in table 2 and fig. 3. The Si/A1 ratio of ZSM-5 crystals formed in the later stages of synthesis may be different from those formed earlier.The A1 content of the crystalline product of the Si/Al = 35 batch was also examined by AES. The Si/Al ratio of ZSM-5 has been plotted against time with Ar+ ion bombardment as shown in fig. 4. Auger electron spectra taken before and after depth profiling yielded an Si/Al ratio of ca. 30-25, indicating that the Si/Al ratio is homogeneous either in the interior or on the surface of crystalline ZSM-5. However, the Si/Al ratio of individual crystals may be different between particles, as shown by EPMA measurements of the gel-free crystals (fig. 3).J-C. Lin and K-J. Chao 2649 5/i LO a I I I I I 20 LO 60 80 100 sputtered time/min Fig. 4. AES measurement on the Ar-ion sputtered ZSM-50(35).Furthermore, we prepared two samples of the same composition. After 8 days at 167 "C, one of the samples was cooled slowly from 167 "C to room temperature in 2 days (S); the other was cooled immediately by quenching the vessel in cold water as reported earlier (F). The average bulk %/A1 ratios of the solid products were 25 for S and 23 for F by ICP. The products were in polycrystalline aggregates of ca. 3-5 pm as observed by electron microscopy. A surface Si/Al ratio of ca. 200-600 was found for sample S and the typical Si/Al ratio of F was ca. 20-30 by analysing five crystals in each case. The silica concentration of the liquid residue of S is lower than that of F, but both have similar aluminium contents. This indicates that the solid cluster of S may be composed either of a thin skin of siliceous material coating the polycrystalline aggregate ZSM-5 or of crystals similar to sample F stacked with small ones of high %/A1 ratio formed during the cooling stage.Perhaps this would explain the different Si/A1 ratios obtained by Hughes et ~ 1 . ~ on their dispersed powder and pelletized ZSM-5. Pelletizing the zeolite breaks up the aggregates and the interior of the aggregates is exposed to the analysis by XPS. Thus, the aluminium distribution of ZSM-5 may be affected by the method of preparation. In summary, we found a homogeneous distribution of aluminium in the individual gel-free ZSM-5 crystals, large or small. In the same batch product, the Si/A1 ratio of each crystal can be different. References 1 S. L. Suib, G. D. Stucky and R. J. Blattner, J . Catal., 1980, 65, 174. 2 J. Dwyer, F. R. Fitch, F. Machado, G. Qun, S. M. Smith and J. C. Vikerman, J . Chem. SOC., Chem. 3 A. E. Hughes, K. G. Wilshier, B. A. Sexton and R. Smart, J . Catal., 1983, 80, 221. 4 R. von Ballmoos and W. M. Meier, Nature (London), 1981, 289, 78. 5 (a) E. G. Derouane, J. P. Gilson, Z. Gabelica, C. Mousty-Desbuquoit and J. Verbist, J . Catal., 1981,71, 447; (b) E. G. Derouane, S. Detremmerie, Z. Gabelica and N. Blom, Appl. Catal., 1981, 1, 20. 6 C. E. Lyman, P. W. Betteridge and E. F. Moran, ACS Sym. Ser., 1983, 218, 199. 7 K. J. Chao, T. C. Tsai, M. S. Chen and I. Wang, J. Chem. SOC., Faraday Trans. 1, 1981, 77, 547. 8 Handbook of Auger Electron Spectroscopy (Physical Electronics Industries, Eden Prairie, Minnesota, 9 K. J. Chao, J. C. Lin and Y . Wang, Zeolite, 1986, 6, 35. Commun., 1981, 422. 1976). Paper 5 / 15 18 ; Received 4th September, 1985 88 FAR 1