nanocrystalline zeolite y: synthesis and characterization - iopscience

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Nanocrystalline Zeolite Y: Synthesis andCharacterizationTo cite this article: Niken Taufiqurrahmi et al 2011 IOP Conf. Ser.: Mater. Sci. Eng. 17 012030

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Nanocrystalline Zeolite Y: Synthesis and Characterization

Niken Taufiqurrahmi, Abdul Rahman Mohamed and Subhash Bhatia School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 Nibong Tebal, Pulau Pinang,Malaysia Email: [email protected] Abstract Nanocrystalline zeolite has received significant attention in the catalysis community. Zeolites with a crystal size smaller than 100 nm are the potential replacement for existing zeolite catalysts due to its unique features with added advantages. Zeolite FAU type Y is one of the most studied framework of all zeolites, and has been used as catalysts for number of reactions in the refinery and petrochemical industry. The present paper covers the synthesis of nanocrystalline zeolite Y under hydrothermal conditions from clear synthesis mixtures. The crystal size of zeolite Y is influenced by temperature, aging time, alkalinity, and water content. The synthesized Y is characterized by X-ray diffraction (XRD), Fourier Transmission Infrared Sprectroscopy (FTIR), Transmission Electron Microscope (TEM), Scanning Electron Microscope (SEM) and Nitrogen Adsorption.

1. Introduction Zeolites are crystalline microporous aluminosilicates with open framework structure of three-dimensional tetrahedral units generating a network of pores and cavities having molecular dimensions. These materials are widely used in applications, such as separations, catalysis, ion exchange, and adsorption due to their unique properties such as molecular sieving, high thermal stability, acidity, adsorption capacities, shape selectivity and ion exchange [1]. It is generally recognized that particle sizes of zeolites have a significant influence on the catalytic performance, particularly in reactions involving the external surface and controlled by a diffusion process. Recently, nano-sized zeolite crystals have received attention due to their unique physicochemical properties. Nanocrystalline zeolites are zeolites with discrete, uniform crystals with dimensions of less than 100 nm that have unique properties relative to conventional micrometer-sized zeolite crystals [2].

Moreover, several research groups have devised synthesis conditions that yield small zeolite crystals [3-7]. Nanocrystalline zeolites have higher external surface areas and reduced diffusion path lengths relative to conventional micrometer-sized zeolites, which made them promising catalytic materials and adsorbents.

As the particle size decreases, there is an increased ratio of atoms at or near the surface relative to the number of atoms on the inner part of the crystal. Therefore, ratio of external to internal number of nanocrystalline zeolite atoms increases rapidly [8].

Zeolite Y is a faujasite molecular sieve with 7.4 Å diameter pores and a three-dimensional pore structure [1]. The basic structural units for zeolites Y are the sodalite cages, which are arranged so as to form supercages that are large enough to accommodate spheres with 1.2 nm diameter. The primary application for Y zeolites has been in catalytic cracking of petroleum into gasoline range hydrocarbons. Among the zeolites used in industrial scale, zeolite Y or faujasite including ultra-stable Y zeolite (USY) is the most widely employed materials. It is the main component of fluid catalytic cracking (FCC) catalyst at a volume above 100,000 tons/yr like refining processing in petrochemical industries [5].

CAMAN IOP PublishingIOP Conf. Series: Materials Science and Engineering 17 (2011) 012030 doi:10.1088/1757-899X/17/1/012030

Published under licence by IOP Publishing Ltd 1

Zeolite Y, with nanoscale crystal size has larger intercrystalline void space, larger pore volume, and more external surface acid sites, and it exhibits higher activity, and better stability, which attracts a growing interest in the synthesis and application of nano-zeolites [9]. The reduction in the crystal size of zeolite Y resulted in an increase activity and selectivity in Fluidized Catalytic Cracking (FCC) due to improved diffusion of reactants and products. In catalytic cracking of gas oil using zeolite Y as the catalyst, smaller crystal sizes produced more gasoline and light gasoil with better hydrothermal stability than bigger crystal [10,11]. Nanocrystalline Y zeolites may be particularly useful in environmental applications as a NOx storage-reduction (NSR) due to the small crystal size and large internal and external surface areas [12].

In this present study, the synthesis of nanocrystalline zeolite Y is reported under hydrothermal conditions from clear synthesis mixtures. The variables affecting the crystal size of Y are temperature, aging time, alkalinity, and the water content [4,6]. The decrease in the crystal size to the nanoscale range produced substantial changes in the physicochemical properties of zeolite. The nanocrystalline zeolite Y was characterized using X-ray diffraction (XRD), Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), Fourier Transform Infra Red (FTIR), and Nitrogen adsorption.

2. Experimental 2.1 Chemicals

Sodium hydroxide (NaOH) pellets, tetraethoxysilane (TEOS), aluminium iso-propoxide (AlP), tetramethylammonium hydroxide (TMAOH, 25% aq.), were purchased from Merck and were used without further purification. 2.2 Preparation of catalysts

The preparation of clear synthesis solution for nanocrystalline zeolite Y followed the method reported by Mintova et al [4,6]. A solution of NaOH 0.05 N was diluted with deionized water. After that, tetramethylammonium hydroxide solution and aluminium isopropoxide, were added in that order, and stirred vigorously until the solution became clear. Tetraethylorthosilicate was added drop wise to the clear solution. This final mixture was aged for 3 days under vigorous stirring at room temperature. The final molar composition was: 0.72(TMA)2O : 0.0094 Na20 : 0.29 Al203 : 1 Si02 : 108.82 H20.

The crystallization of zeolite was performed in a Teflon-lined stainless steel autoclave. After being filled with the prepared clear solution, the autoclave was completely sealed and heated at 100 °C and crystallization time was 6 days. The solid product was recovered by centrifugation at the speed of 20,000 rpm for 1 h, washed several times with distilled water, dried over night at 80 °C, and calcined in air at 550 °C for 8 h.

2.3 Characterization

The synthesized nanocrystalline Y zeolite and commercial Y zeolite (from Zeolyst) were characterized by several different techniques, including X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), and Nitrogen adsorption. The powder XRD patterns of catalyst were obtained using XRD measurement on a Philips diffractometer with Cu target Kα-ray. The analysis was conducted at 2 theta values of 10–50o. The particle size and morphology of the microcrystalline and nanocrystalline zeolite were analyzed using SEM and TEM image. For SEM, the powder form samples were carefully put on a double-sided tape with the aluminum stub as the base. The observations were made at different magnifications using a Leica Cambridge S-360SEM and JEOL scanning electron microscope. For TEM, the powder sample was suspended in 100 % ethanol under ultrasonic treatment for 15 min. Several drops of ethanol containing the lightest powder sample were deposited on a copper grid. The TEM images were recorded using a Philip CM12 transmission electron microscope operated at 80 kV. The IR spectra was scanned using a Perkin-Elmer FTIR (Model 2000) in the wavelength range of 400 to 4000 cm-1 with KBr pellets method. Nitrogen gas adsorption measurement was performed at 77 K on a Micromeritics ASAP 2001. The sample was first degassed for 3 h under vacuum at 300oC prior to the analysis.


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3. Results and Discussion 3.1 XRD

The XRD pattern of the nanocrystalline zeolite Y synthesized is shown in Figure 1.a. The XRD pattern of the large crystals, commercial zeolite Y is used as a reference (Figure 1.b). The XRD pattern of the sample is identical to that of the reference, indicating the FAU structure of the sample. There is a clear broadening of the reflections from the sample and the decrease in peak intensity, which is attributed to the presence of small crystals.

Figure 1. XRD patterns of (a) nanocrystalline zeolite Y and (b) microcrystalline zeolite Y 3.2 SEM and TEM analysis

In order to establish the crystal size of the zeolite samples, the scanning electron micrograph (SEM) technique was used. Representative micrographs of the microcrystalline zeolite Y and nanocrystalline zeolite Y are shown in Figure 2a and 2b, respectively. Large crystal size around 500 nm is shown for microcrystalline zeolite Y image. A small average crystal size of about 50 nm was observed and it is also seen that the crystal size distribution appears to be uniform. This is expected since the precursors are protected from aggregation during the crystallization.

(a)

(b)

Figure 2. (a) SEM image of microcrystalline zeolite Y (b) SEM image of nanocrystalline zeolite Y


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Figure 3 shows the TEM images of nanocrystalline depicting its morphology. A small average crystal of about 50 nm was observed in Figure 3a and porosity of nanocrystalline zeolite Y is shown in Figure 3b.

(a) Magnification 110,000 x

(b) Magnification 180,000 x

Figure 3. TEM image of nanocrystalline Zeolite Y

3.3 FTIR Analysis

FTIR spectra are shown in Fig. 4a and 4b for nanocrystalline zeolite and microcrystalline zeolite respectively. Peak position is nearly identical for the two samples. The peak at 460 cm-1 is assigned to the structure insensitive internal TO4 (T = Si or Al) tetrahedral bending peak of zeolite Y. The peak at 565 cm-1

is attributed to the double ring external linkage peak assigned to zeolite Y. The peaks at 685

and 775 cm-1

are assigned to external linkage symmetrical stretching and internal tetrahedral symmetrical stretching respectively. Furthermore, the peaks at 1010 cm-1 and 1080 cm-1

are assigned

to internal tetrahedral asymmetrical stretching and external linkage asymmetrical stretching respectively and peak around 3400 cm-1 is assigned to hydroxyl groups of zeolite. Overall, the FTIR spectra of synthesized nanocrystalline zeolite Y is matched well with the typical FTIR absorption peaks of microcrystalline zeolite Y [7].

Figure 4. (a) FTIR Pattern of nanocrystalline zeolite Y and (b) FTIR pattern of microcrystalline

zeolite Y


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3.4 Nitrogen adsorption Analysis The structure properties of both microcrystalline and nanocrystalline zeolite Y were obtained from

the nitrogen adsorption isotherms and are listed in Table 1. The surface area can be observed of decrease in crystal size from the very large increase in external surface area. This is based on the fact that as the size of the crystal for the same quantity (x g) of the sample decreases, the total external surface area increases [8].

Table 1. N2 Adsorption desorption result

Microcrystalline zeolite Y

Nanocrystalline zeolite Y

BET Surface Area (m²/g) 648 1126

External Surface Area (m²/g) 44 583*

Micropore surface area (m²/g) 604 543

Median pore width Å -‐ 5.178 t-‐plot micropore volume (cm3/g) 0.28 0.214

* Obtained from assynthesized nanocrystalline zeolite sample

The total surface area of commercial zeolite Y was 648 m2/g, with external surface area 44 m2/g.

Nitrogen adsorption isotherms for the synthesized nanocrystalline Y samples were measured both before (as-synthesized) and after calcination. Table 1 shows that the total surface area of the nanocrystalline zeolite sample is very high, which is 1126 m2 /g. The total surface area obtained from the calcined samples contains contributions from both the internal and the external surfaces. The external surface areas of nanocrystalline zeolite sample was obtained from the total surface area of the as-synthesized samples in which the internal surface is blocked by template molecules represents the external surface area of the zeolite sample [12]. The external surface area is 583 m2 /g, around 51 % from total surface area, which implies the presence of nanozeolites in the sample.

Figure 5 and 6 show the nitrogen adsorption isotherms of the microcrystalline zeolite and calcined nanocrystalline zeolite Y, respectively. For nanocrystalline zeolite Y sample, the initial adsorption step at low relative pressure indicates complete filling of the micropores, with median pore width of 5.178 Å and limiting micropore volume of 0.214 cm3 were determined by the Horvath-Kawazoe and t-plot method, respectively.

Figure 5. N2 adsorption of microcrystalline zeolite Y


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Figure 6. N2 adsorption of nanocrystalline zeolite Y

4. Conclusion Synthesis of nanocrystalline zeolite Y has been successful using hydrothermal synthesis from clear synthesis mixtures. The decrease in the crystal size to the nanoscale range produced changes in the physicochemical properties of the zeolite compare to microcrystalline zeolite. Acknowledgements The authors gratefully acknowledge the financial support provided by the Ministry of Science, Technology and Innovation, Malaysia under e Science fund (Project: 03-01-05-SF0221). References [1] S. Bhatia, Zeolite catalysis: principles and applications. CRC Press. FL (USA), 1990: p. 7-12. [2] S.C. Larsen, Nanocrystalline zeolite and zeolite structures: synthesis, characterization, and

application. Journal of Physical Chemistry C, 2007. 111: p. 18464 -18474. [3] L. Tosheva, and V.P. Valtchev, Nanozeolites: synthesis, crystallization mechanism, and

application. Chemistry of Materials, 2005. 17: p. 2494- 2513. [4] S. Mintova, N. H. Olson, and T. Bein. Electron Microscopy Reveals the Nucleation Mechanism

of Zeolite Y from Precursor Colloids. Angewandte Chemie - International Edition. 1999, 38: p. 3201-3204.

[5] Y. C. Kim, J. Y. Jeong, J. Y. Hwang, S. D. Kim, W. J. Kim. Influencing factors on rapid crystallization of high silica nano-sized zeolite Y without organic template under atmospheric pressure. Journal of Porous Materials. 2008, 16: p. 299-306.

[6] O. Larlus, S. Mintova, T. Bein. Environmental syntheses of nanosized zeolites with high yield and monomodal particle size distribution. Microporous and Mesoporous Materials, 2006. 96: p. 405–412.

[7] Holmberg, B.A. H. Wang, J.M. Norbeck, and Y. Yan, Controlling size and yield of zeolite Y nanocrystals using TMABr. Microporous and Mesoporous Materials, 2003. 59: p. 13-28.

[8] M. Singh, K. Raviraj, and N. Viswanadham, Effect of crystal Size on physico-chemical properties of ZSM-5. Catalysis Letter, 2008. 120: p. 288-293.

[9] W., Song, V.H., Grassian, and S.C. Larsen, Development of improved materials for environmental applications: nanocrystalline NaY zeolites. Environmental Science & Technology, 2005. 39: p. 1214-1220.

[10] C.J.H., Jacobsen, C., Madsen, T.V.W., Janssens, H.J., Jakobsen, and J. Skibsted, Zeolites by confined space synthesis-characterization of the acid sites in nanosized ZSM-5 by ammonia desorption and Al/SiMAS NMR spectroscopy. Microporous and Mesoporous Materials, 2000. 39: p. 399-401.

[11] M.A. Camblor, A. Corma, A. Martinez, F.A. Mocholi and J.P. Pariente. Catalytic cracking of gasoil: Benefits in activity and selectivity of small Y zeolite crystallites stabilized by a higher silicon-to-aluminium ratio by synthesis. Applied Cataysis, 1989. 55: p. 65-74.

[12] G. Li, C. A. Jones, V. H. Grassian, S. C. Larsen. Selective catalytic reduction of NO2 with urea in nanocrystalline NaY zeolite, Journal of Catalysis, 2005. 234: p. 401–413.


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nanocrystalline zeolite y: synthesis and characterization - iopscience

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