Synthesis and characterization of a novel lamellar type ... - NOPR

Indi an Jo urnal of C he mi stry Vol. 43A, December 2004, pp. 2514-2517

Synthesis and characterization of a novel lamellar type aluminosilicate molecular sieves, NCL-5 N Venkatathri Catalysis di vis io n, Nat io nal C he mi ca l Laboratory, Pune 4 11 008, India

Received 30 Decell/ber 2003; revised 5 Allg lls/ 2004 A nove l lamellar ty pe a lumin os ili ca te mo lec ul a r s ie ve, NCL-5 ha s bee n sy nthes ized and characterized us in g various phys icoc he mi cal tec hniqu es s uc h as XRD , SEM. TG/ DTA, Fr-IR and MAS NMR. The res ult s have been compa red with SiMC M-41 sa mple sy nthes ized usin g th e sa me te mplate (cety ltrime th y l ammon ium bro mid e) . XRD a nal ys is sho ws that NC L5 is a no ve l short ran ge orde red aluminosilicate. SEM shows that it is nanocry stalline w ith phase purity . TG/ DTA anal ys is shows that th e NCL-5 sa mpl e ox id at ive ly deco mposes the occ luded te mplate in a s ing le ste p whil e Si-MCM-4 1 ox idati ve ly deco mposes in two different steps. This may due to th e nanoc rysta llinit y o f the forme r sampl e. A luminiu m MAS NMR shows th e prese nce of ma x imum oc tahedral co-ordinated a luminium in NCL-5 samp le whil e si licon NMR shows the prese nce of three different types o f sili co n. 7

IPe Code: Int. C I. BOlJ 29/06

The discovery of a new family of mesoporou s molecular sieves called M41S was reported by researchers at Mobil R&D corporation t. The influence of su rfactant/si lica molar ratio (S ur/Si) in M4lS sy nthes is was studied in the simpl e sy nthesis sy~tem cons isti ng of tetraorthoethy I orthosi Iicate (TEOS), water and cetyltrimethy lammonium (CTMA) cation at 100°C. As the Sur/Si increased from 0.5 to 2, th e silicio us products obtained were identified and could be classified into four gro upS2: MCM-41 (hexagonal ), MCM-48(cubic), thermally un stabl e M41 S and a molecu lar species, the o rgan ic octamer ((CTMA )Si0 25 k One of the thermally un stable s tructures has been identified as a la me llar phase. These results are cons isten t with micellar phase transformations that occ ur at va ri o us surfactant conce ntrations and re i nforce the concept that mice ll e s tru ctures serve as templating age nts for the formation of M41 S type material s. [n th is pape r, in cont inuati o n of ear li er work 3A a no ve l la mellar type a luminosilicate molecular s ieves, NCL-5 was synthesi zed and characteri zed by variou s phys icochemical techniques suc h as XRD , SEM , TG/DTA, C & N analysis, FT-IR and MAS NMR. The res ults have bee n compared with Si-MCM-41 samples synthesized usi ng the same te mplate.

Materials and Methods SyntheSis of Si-MCM-41

A Si ana log of MCM-4 I was prepared hydrothe rmally using a ge l with the following molar

compOSItIOn In term s of ox ides: Si02 : 0.27 (CTMA hO: 0.13 (TMA h O: 60 H 2 0 Sodium sili cate (28.48% Si02 , 9.03 % Na20. 62.5 % H 2 0), cetyltrimeth y lammonium bromide (99%, Aldrich , USA), tetramethylammonium hydroxide (TMAOH, 25 % aqueous so luti on , Aldrich) and fum ed sili ca (Cab-O-Sil , 99 %, Flu ka, USA) were used in th e sy nth es is. [n a typical sy nthesi s TMAOH ( 18. 9 g) was added to sodium sili cate (16.86 g) diluted wi th 100 g wate r. In another beaker CTMABr ( 19.68 g) was di sso lved in 50 g water and 30 g ethan o l and aqueous ammonia ( 1.9 g) we re added to it (so luti on A) . So lution A was added to th e above mi xture of sod ium si licate and TMAOH and then fumed si li ca (7.06 g) was added to it. The mixture was stirred for I h. The gel formed (pH 11.5 ) was th e n transferred to an autoc lave and heated at 110°C fo r 5 day s. The product was th e n filtered, washed with distill ed water, and dri ed at ambient temperature. The samp le was calcined at 540°C in nitrogen flo w fo r 1 h and th e n in air for 6h. Synthesis of la mellar type aiuminosilicatc, NCL -5

NCL-5 sumples prepared hydrothermally usin g a ge l with th e follow ing molar compositio ns in term s of ox icles, Si0 2:O.02 AbO}:O.27 (CTMA)20 : 0.1 3 (TMA)20:xN a20:60 H2)' Th e pre paration procedure was s imi lar to that o f Si-MCM-41 except ing th at sodium hydroxide (0.035 g) di sso lved in 8 g water was added to the mixture of sod ium sili cate and TMAOH and a ca lculated qu antity of AI 2 (SO~ )".

VENKATATHRI: NOVEL LAMELLAR TYPE ALUMINOSILICATE MOLECULAR SIEVES

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18 H20 (2.S2 g) dissolved in 30 g water was added to the final gel and stirred well. The gel was crystallized as described earlier (l1O°C for S days). The sample was calcined at SSO°C in air for 6 h. Characterization

The sample synthesized during the course of the work was analyzed for qualitative identification by Xray powder diffraction (Rigaku, Model D/MAX III YC, Japan; Ni filtered Cu-Ka radiation, A = I .S404A.; graphite crystal monochrometer; computer controlled automated diffractometer). The morphologies of the aluminophosphate synthesized was investigated using a scanning electron microscope (JEOL, JSM S200). The framework region (400-1300 cm· l ) of the synthesized aluminophosphates were analyzed using a Nicolet 60SXB Ff-IR instrument in the diffuse reflectance mode usi ng a 1:300 ratio of the sample to KBr mixture. Simultaneous TGIDT A analysis of the crystalline phases was performed on an automatic derivatograph (Setaram TG-DT A 92). The thermograms were reccorded in flow of nitrogen with heating rate 10 Klmin. MAS NMR spectra were recorded in the solid state with a Bruker DRX SOO spectrometer operating at a field of 11.7 Tesla. 27 Al spectra were recorded at a frequency of 130.3MHz, with a pulse length of 2fJ.s and a spinning speed of 3-S KHz. 29Si NMR spectra were recorded at 99.3 MHz, 2fJ.s (4S0) pulse width and repetition time of 2s. Tetramethylsilane (for silicon) and 1M AI(NO))3 solution (for aluminium) were used as standards.

Results and Discussion The XRD pattern of the as-sy nthesized Si-MCM-41 is similar to those reported 5 for Si-MCM-41 samples. However, the NCL-S XRD pattern did not match with any of the reported materials. The X-ray differaction pattern of the as-synthesized lamellar material (NCL-S) exhibited a high intensity peak having d-spacing of approximately 24.39 A. and three high angle peaks having d spacings (12.5S, 8.48 and 6.37 A.) consistent with lamellar indexing of 001 reflections as shown in Fig. 1b. Upon calcination at 5S0°C, the X-ray diffraction was featureless. The lamellar phase could be represented by sheets or bilayers of surfactant molecules with the hydrophilic ends pointed towards the oil-water interface, while the hydrophobic ends of the surfactant molecules face one another. Any silicate structures produced from this liquid crystal phase would be similar to that of two dimensional layered

Fig. l-X-ray diffraction pattern of (a) Si -MCM -41. and (b) NCL-S.

materials such as magadiite or kenyaite. However, the lack of any observable peaks in the XRD pattern of the lamellar material in the region of 20-2S o 28 suggests that these silicate layers of this lamellar phase are not well ordered as those of layered silicates. This lack of order may be due to the higher concentration of silanol groups resulting in less condensation of the silicate species as suggested by the NMR data. Removal of the surfactant from between the silicate sheets could result in a condensation of layers, collapsing any structure and forming a dense phase with little structural order or porosity. Elemental analysis gave the Sil AI ratio of the NCL-S is 23.0. The morphology of the samples as observed by SEM (Fig. 2) image reveals that the sample is highly crystalline without any amorphous material. Both the samples Si-MCM-41 and NCL-5 are having spherical morphology with 6.36 fJ.m and 0.25 fJ.m particle sizes, respectively. The TG-DTA curves of the template containing NCL-S and Si-MCM-41 samples are presented in Fig. 3. In the low temperature region up to 138.5 and IS6.7 °C there is an endothermic weight loss of 6.1 and 7.6% mainly due to the loss of adsorped water. In the temperature regions of 138 .S to 650.4 and 156.7 to 666.8°C, there are one and two stages of exothermic weight loss (4S% and 49.2%, 15.6%) due to the oxidative decomposition of the organic template occluded in the sample. Single stage oxidative decomposition in NCL-5, may be due to the nanocrystalline morphology of the sample. Similar results were also reported for nanocrystalline ZSM-35 sample 3. In the IS6.7 to 293.4°C region , occluded template molecules In the pores oxidati vely

INDIAN J CHEM, SEC A, DECEMBER 2004

2516

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Fig. 3-TG(A)/DTACB) analysis of (a) S i-MCM-41 and (b) NCL-5

Fig. 2-Scannin g electron microscopic picture of (a) Si-MCM-41 and (b) NCL-5

decompose with probably a part of the hydrogen burning. While in the region 293.4 to 666.8°C, the coke is elminated by oxidation. Based on the total loss due to template removal and the carbon and nitrogen analysis (C = 29.43 %, N = 2.08 % for NCL-5) and (C = 44.59%, N = 2.55 % for Si-MCM-41) respectively, 0. 149, 0.182 template molecule per 100 g of the sample is calcu lated . The FT-IR spectrum of the as-synthes ized SiMCM-4 1 and NCL-5 in the framework region is shown in Fig. 4. The different modes of tetrahedral linkages give bands at 1215(m), 1059(vs), 964(m), 912(w), 785(m), 719(m), 604(w) and 448 (s). The bands can be assigned to asymmetric stretching, symmetric stretching, pore opening and bending

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Fig. 4-Framework region FT-IR spectra of (a) Si -MCM-4l and (b) NCL-5

vibrations of T-O-T linkages as des cribed in earlier literature 6 . The difference between the spectra of Si-MCM-41 and NCL-5 is that the peaks at 1215 (m) and 604(w) are stronger in the case of Si-MCM-41. The 27 AI MAS NMR spectra of as-synthesized NCL-5 are presented in Fig. Sa. The sample shows a

VENKATATHRI: NOVEL LAMELLAR TYPE ALUMINOSILICATE MOLECULAR SIEVES

2517

ppm are due to side bands. The results were in contradiction with the results of AI-MCM-41 samples 7 , where aluminium was found in tetrahedral co-ordination. 29 Si MAS NMR spectra of assynthesized NCL-5 samples show 3 distinct lines at -90, 98 and 110 ppm attributed to Q2, Q3 and Q4 species respectivell.

Acknowledgement The author thanks CSIR, New Delhi for a Research Associate fellowship .

References I (a)

2

-so ,,;tm Fig. 5_27 AI (a) and 29Si (b) MAS NMR spectra of NCL-5

single signal with 8 = 1.986 ppm due to Al In octahedral co-ordination. The other weak signals at -34 and -38 ppm are due to non-ocatahedral co-ordination. The remaining two signals at 20, -58

3 4 5 4 5 6

Krege C T, Leonowicz M E, Roth W J, Vartuli J C & Beck J S, Nature, 359 ( 1992) 710. Vartuli J C, Schmitt K D, Kresge C T , Roth W J, Leonowicz M E, McCullen S B, Hellring S D, Beck J S, Schlenker J C, Olson D H & Sheppard E W, Stud Surf Sci Catal, 84 (1994) 53. Venkatathri N, Indian J Chern, 43A (2004) 2315. Venkatathri N, Indian J Chern, 43A (2004) 2325 . Venkatathri N, 14'11 International Zeolite Conference, Cape Town, South Africa, April (2004) 189. Jansen J C, van der Gaag F J & van Bekkum H, Zeolites, 4 (1984) 369. Chaudhari K, Das T K, Chandwadkar A J & Sivasanker S, J Catal, 186 (1999) 81. Gontier S & Tuel A, Stud Surf Sci Catal, 105 (1997) 29.