Synthesis and characterization of the nanocrystalline zeolite ZSM-35

Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) © 2004 Elsevier B.V. All rights reserved.

189

SYNTHESIS AND CHARACTERIZATION OF THE NANOCRYSTALLINE ZEOLITE ZSM-35 Venkatathri, N. Catalysis Division, National Chemical Laboratory, Pune 411 008, India. Tel/Fax: +91-20-5893761. E-mail: [email protected]

ABSTRACT Nanocrystalline ZSM-35 zeolite with 200nm particle size compared to 6 x 17 |Lim size of the parent zeolite was synthesized by reducing the crystallization period. Analysis of the product by XRD, SEM, reveals the samples were highly crystalline. TG/DTA techniques shows that the Nanocrystalline sample template decomposes in a single stage compared to the three stage decomposition in parent one. FT-IR in the framework region shows that the difference in intensity of the 550 cm'^ band. In the -OH region similar to ZSM-35, nanocrystalline sample shows three bands. How ever the intensities were low. ^^Si MASNMR reveals the presence of single peak for Si: [4Si] species in nanocrystalline samples compared to the ZSM-35 samples with two peaks for Si:[3Si: Al] and Si: [4Si] species, n-hexane hydroisomerization reaction shows that Nanocrystalline ZSM-35 is a active and selective catalyst compared to ZSM-35. Keywords: Nanocrystalline; ZSM-35; XRD; SEM; TG/DTA; FT-IR; MASNMR INTRODUCTION Zeolites are crystalline aluminosilicates with rigid three-dimensional framework of [Si04] and [AIO4] tetrahedra linked to each other by corner sharing of oxygen ions. Each [AIO4] tetrahedron creates a negative charge on the framework of zeolite (Breck 1969). The framework negative charge is balanced by cations, which occupy non-framework exchangeable positions. Zeolite were crystallized in presence of an organic template and a source of aluminium, silicon, alkali and a solvent at temperatures range in between 100-200°C. Crystallization carried out as follows. Initially a small nuclei was formed by reaction with aluminium with silicon. This nuclei grow into a smaller crystals around the template. Then into bigger crystals (Lowenstein 1954). Arresting the reaction in the first step when it grow in to small crystals give small particles gives more surface area resuhs more catalytic activity especially in hydroisomerization reactions. ZSM-35 was originally patented by Mobil in 1977 (Plank et al. 1977). Even though it is isostructural with ferrierite but has a significantly shorter a-axis (1.916 nm for ferrierite compared with 1.892 nm for ZSM-35) (Kokotailo et al 1985). Its structure consists of eight membered rings intersecting ten membered rings with pore sizes of (4.2 x 5.4) A and (3.5 x 4.8) A (Meier et al 1987). Although the catalytic applications of ZSM-35 have been published in detail (Kibby et al 1974; Xu et al 1995; Traa et al 1997; Lee et al 2000; Neyestanaki et al 2000; Traa et al 2000; Byggningsbacka et al 1998; Kwak et al 1997; Xu et al 1995;Wenyang et al 1989), only few studies [Wenyang et al 1989; Borade et al 1994; Venkatathri et al 2002) have been carried out on the synthesis and characterization. In the present work, Nano sized crystals were synthesized by shortening the crystallization period to 5 days instead of lOdays. The product was analyzed by various physicochemical techniques such as XRD, SEM, Carbon and Nitrogen analysis, TG/DTA, MASNMR and FT-IR techniques. The catalytic activity tested for n-hexane hydroisomerization reaction. Fully crystallized ZSM-35 were taken for the comparison studies. EXPERIMENTAL Synthesis of Nanocrystalline ZSM-35 In a typical procedure to synthesize Nanocrystalline ZSM-35, 3.3g of sodium aluminate (99%, s.d.fme, India) was mixed well with 0.7g of sodium hydroxide (99%, s.d.fme, India) and 129g of distilled water. This mixture was stirred well until a clear solution was obtained (solution A). Another solution B was made by

190 thorough mixing of 46.47g of silica sol (30%) and 18.3g of ethylene diamine (EDA, 99%, Aldrich, U.S.A). Solution A and B were mixed well to make a clear mixture and charged into a teflon lined steel autoclave. Crystallization was carried out at 177°C for 5 days. The product was removed and washed with deionised water and the sample was dried at 110°C for 24h. It was subjected to various physicochemical characterizations. Synthesis carried out for 1 Odays and the sample was taken for comparison purpose. Characterization X-ray diffraction patterns were recorded on a Rigaku (D/MAX III VC) instrument in the 20 region of 5-45°. Scanning electron microscope pictures were taken using a JEOL JSM 5200 microscope: chemical analysis was carried out by XRF using a Rigaku 3070 X-ray Spectrometer. Carbon and nitrogen were estimated by microanalysis. The framework IR spectra were recorded in the diffuse reflectance mode using 5% sample in KBr (Nicolet 60SXB). To study the nature of the surface hydroxy 1 groups in the transmittance mode, self-supported wafers (~10mg/cm"^) and an IR cell with controlled environment chamber were used. The sample was activated in situ at 673K under vacuum (10"^ torr) for 4h and then cooled to 323K before recording the spectrum (4cm"^ resolution, averaged over 500 scans). ^^Al and ^^Si MAS NMR spectra were recorded on a Bruker MSL 300 spectrometer. ^^Si NMR spectra were recorded at 59.6 MHz, 2|LIS (45°) pulse width and repetition time of 2s. ^^Al NMR spectra were recorded at 78.2 MHz, 1|LIS pulse width and 500 ms repetition time. Tetramethylsilane (for silicon) and IM A1(N03)3 solution (for aluminium) were used as standards. The zeolites were impregnated with a minimum amount of an aqueous solution of Pt(NH3)4Cl2, in order to obtain a Pt-loading of 0.3%, dried in air at room temperature, for at least 48h, calcined in flowing O2 (60 ml min"^) at 400°C for 2h and reduced in flowing H2 (100ml min"^) at 400°C for 3h. The hydroconversion of n-hexane was carried out in a flxed bed down flow tubular glass reactor at atmospheric pressure in the temperature 648K, WHSV (h"^) range 1 and H2/hydrocarbon (mol) ratio 6. The catalyst powder was pelletized, sieved to 10-20 mesh size and 2g of it was loaded into the reactor. Before activity measurements the catalyst was activated at 648K in flowing hydrogen (25ml min"^) for Ih. The reactants were fed using a syringe pump (Sage instruments, USA). The reaction products were analysed using a Hewlett Packard gas chromatograph (5880A) with flame ionization detector and a capillary column (50m x 0.2mm; HPl, cross linked methyl silicon gum). The isomeric products from n-hexane were identified by GC-MS. RESULTS AND DISCUSSION The XRD pattern of Nanocrystalline ZSM-35 and ZSM-35 were simillar (Fig. 1). As-synthesized Nanocrystalline sample shows the partial amorphous nature in the 20 region from 20-30°. However the calcined sample were indicate the full crystallinity. Both were synthesized from the gel of composition 1.85Na20: AI2O3: 15.2Si02: 592H2O: 19.7EDA (Plank et al 1977). Nanocrystalline sample was synthesized at 5 days compared to the ZSM-35 sample in ten days. Elemental analysis of the calcined samples gave the chemical composition as 1.75Na20 [AI2O3: 22Si02]. The elemental composition of both the samples were same. The morphology of the Nanocrystalline sample was spherical particles with 0.2|Lim particle size, where as ZSM-35 samples were granular shapped (Fig. 2) with size 6 x 1 7 jim.

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20

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Figure 1. X-ray diffraction pattern of a) ZSM-35 and b) Nanocrysstalline ZSM-35.

191

Figure 2. Scanning electron micrograph of a) ZSM-35 and b) Nanocrystalline ZSM-35. The TG/DTA curves of the template containing nanocrystalline ZSM-35 and ZSM-35 sample are presented in Fig. 3. The main difference between the two TG/DTA is that the nanocrystalline samples were loose all the template in two steps, the first endothermic loss at 103°C (7.42% loss) is due to the loss of physisorbed template and water the second exothermic step at 325^C (3.69% loss) is due to the oxidative decomposition of the template. As the particle size is small the decomposition is easier in this case. Where as ZSM-35 samples loss in five stages. In the low temperature region up to 586K, endothermic weight loss occurs in two stages (6.56% and 1.88%)) mainly due to the loss of adsorbed water and template. In the temperature region of 586 to 1086K, there are three stages of exothermic weight loss (1.41%, 4.22%, and 3.59% respectively) due to the oxidative decomposition of the organic template occluded in the sample. The probable oxidation stages as follows: in the 586 - 642 K region occluded template molecules in the pores oxidatively decompose with burning of a part of the hydrocarbon, while in the region 642 - 817 K, protonated template molecules interacting strongly with the framework charge and the fragments of templates decomposed and adsorbed at lower temperatures are partially oxidized leaving behind a coke residue. In the last stage (817 - 1086 K), this coke is eliminated by oxidation. Based on the total loss due to template removal and the carbon and nitrogen analysis ( 2.21%)C, 2.04%)N for nanocrystalline ZSM-35, and 4.40% C, 4.15% N for ZSM-35) it is estimated that three ethylene diamine molecules are occluded in each unit cell of ZSM-35 where as the same reduced to half in nanocrystalline ZSM-35.

0 2D€ 4m 6m g@o i@ao Figure 3. TG(A)/DTA(B) analysis of a)ZSM-35 and b) Nanocrystalline ZSM-35.

192

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ppm ppm Figure 4. ^^Al MASNMR spectra of a) Nanocrystalline ZSM-35 and b) ZSM-35; ^^Si MASNMR spectra of c) Nanocrystalline ZSM-35 and d) ZSM-35. The ^^Al and ^^Si MAS NMR spectra of nanocrystalline ZSM-35 and ZSM-35 are presented in Fig. 4. The ^^Al NMR spectra of the nanocrystalline ZSM-35 and ZSM-35 samples [Fig. 4(a) and (b)] reveal a single symmetrical ^^Al line at 52.80 ppm with respect to A1(H20)6^^ indicating a single tetrahedral aluminium species, confirming the high purity of the samples. The two ^^Si peaks in ZSM-35 (Fig. 4(c) and (d)) at -107.00 and -111.51 ppm are due to the Si present in Si:[3Si:Al] and Si:[4Si] environments respectively. In nanocrystalline ZSM-35 only one peak at 112.45 ppm, for Si:4Si environment was appeared. Small shoulder peaks at 123.63 and 134.20 also appeared for environmentally different Si:4Si species. The FTIR spectrum of the as-synthesized nanocrystalline ZSM-35 and ZSM-35 in the framework region is shown in Fig. 5. Both gives bands for the different modes of tetrahedral linkages give bands at 1232(sh), 1062(vs), 1208(sh), 797(s), 696(w), 581 (s), 528 (sh), 460(sh), 424(s) cm"\ However the nanocrystalline sample gave peak at 700(w) as shoulder peak. The bands can be assigned to asymmetric stretching, symmetric stretching, pore opening and bending vibrations of T-O-T linkages described in earlier literature (imsQnetal 1984).

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Figure 5. FT-IR spectra of a) ZSM-35 and b) Nanocrystalline ZSM-35 in the framework region.

193 The transmittance spectrums of the calcined samples in the -OH stretching region is presented in Fig. 6. Three bands at 3736, 3592 and 3552 cm'^ are noticed for ZSM-35. Nanocrystalline ZSM-35 gave bands at 3733, 3656, and 3637 cm'^ The band at 3736 cm"^ can be assigned to terminal Si-OH groups. The other two bands are assigned to bridging hydroxyl groups (Si-OH-Al) situated in the pores confined by the ten and eight membered rings respectively.

Figure 6. FT-IR spectra of a)ZSM-35 and b) Nanocrystalline ZSM-35 in the Hydroxyl region. The isomerization of n-hexane over Pt supported on acidic zeolites is believed to proceed by a bifunctional mechanism (Giaannetto et al 1986; Leu et al 1991). The metal component aids in increasing the rate of isomeriztion, besides lowering catalyst deactivation, n-alkane molecules are adosrbed at dehydrogenation/hydrogenation sites where n-alkenes are formed. These migrate and interact with acid sites and 2° carbenium ions are generated, which further rearrange to more stable 3° carbenium ions. Finally the 3° carbenium ions are hydrogenated at the metallic sites yielding isoalkanes. The monobranched isomers are formed predominantly as they are the primary products. These undergo consecutive reactions to yield multiply branched isomers (Fameth et al 1995). The activity of n-hexane hydroisomerization over Nanocrystalline ZSM-35 and ZSM-35 were given in Table 1. Nanocrystalline ZSM-35 is more active and selective catalyst was shown by the experiment. In both the cases, 2-methyl pentane formed as the major product and Nanocrystalline sample give this as the only product. How ever variety of products was given by ZSM-35 catalyst. Table 1. n-hexane hydroisomerization on Nanocrystalline ZSM-35 and ZSM-35 molecular sieves. Sample

n-hexane

2-methyl pentane

3-methyl pentane

2,2 dimethyl butane

2,3 dimethyl butaneOO

Nanocrystalline 35.19 64.78 ZSM-35 23.44 ZSM-35 65.77 6.67 3.78 0.81 Conditions: Catalyst - 2g, pt - (0.3%), WHSV (h"0 = 1.0; H2/n-hexane (mole) = 6, and Temperature = 648k, TOS- Ih.

CONCLUSIONS Nanocrystalline ZSM-35 samples were prepared by reducing the time duration (5 days compared to 10 days) of the synthesis of ZSM-35. Presence of some amorphous region were found in nanocrystalline samples. How ever on calcination they were disappeared. Scanning electron microscope proves the nanocrystalline samples were highly crystalline with spherical morphology and 200 nm particle size. TG/DTA shows that nanocrystalline samples loss template in two stages compared to ZSM-35 in five stages. The total weight loss also reduced to half ^^Al MAS NMR shows that both of the samples containing aluminium with tetrahedrally co-ordinated. ^^Si MASNMR shows that the silicon is present in

194 nanocrystalline sample as Si: 4Si species. Also two sholder peak were appeared for environmentally different Si: 4Si species. FT-IR spectroscopy at framework region is simillar in both the case, however the peak 700 cm'^ differ. Hydroxyl region FTIR shows that the presence of one Si-OH peak (3733 cm'^) and two silanol groups at 3656, and 3637 cm'\ N-hexane hydroisomerization reaction reveals that the nanocrystalline sample were highly active selective catalyst compared to ZSM-35 samples. ACKNOWLEDGEMENT The author N.V thanks CSIR, New Delhi for a Research Associate fellowship. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

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