Ind. Eng. Chem. Res. 2009, 48, 4837–4843
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A Novel Approach for the Synthesis of Zeolite Y D. Karami and S. Rohani* Department of Chemical and Biochemical Engineering, The UniVersity of Western Ontario, London, Ontario N6A 5B9, Canada
In this study, a novel approach for the synthesis of zeolite Y is described. The effect of varying the starting silica source on the purity of zeolite Y was investigated, while all other reaction parameters were kept constant. The purity of zeolite was expressed in terms of percent crystallinity obtained from the powder XRD. The silica sources were all powders of varying types (except for silica sol). To study the influence of the varying silica sources on the purity of the resulting zeolite Y, the best gel composition and synthesis condition for one batch were chosen. The gel composition was 4Na2O:1Al2O3:6SiO2:200H2O. The aging temperature and duration of aging were chosen as room temperature and 120 h, respectively. The synthesis temperature and synthesis time were 100 °C and 48 h. The experiments were all conducted in sealed 30 mL Teflon bottles. The extent to which the pure zeolite Y was formed depended on the BET specific surface area of the silica sources (ranging from 150 to 300 m2/g) but not on the synthesis conditions. This novel method for zeolite Y preparation allows one to eliminate the vigorous agitation step required for the preparation of a homogeneous silica solution, thereby simplifying the synthesis of zeolite Y in one single vessel. 1. Introduction Among the zeolites used on an industrial scale today, zeolite Y (faujasite, FAU) is one of the most widely employed materials.1 The major applications of synthetic zeolite Y are in the fields of fluid catalytic cracking (FCC) of vacuum gasoil and in adsorption of volatile organics from wet off-gas streams. On an industrial scale, zeolite Y is commonly prepared with high aluminum content (SiO2/Al2O3 < 5), but in most cases it is used in a silicon-enriched form. The major method to prepare zeolite Y is hydrothermal synthesis, which is the naturally occurring process that produces several classes of inorganic natural minerals such as crystalline silica and zeolites. The hydrothermal synthesis of zeolites can be described by the following chemical reaction:2,3 SiO2+ Al2O3+ Na2O + H2O f aluminosilicate gel f crystalline zeolites In this conventional method, two solutions containing sources of silica and alumina are added together with vigorous agitation. The formed aluminosilicate gel is kept for aging and then crystallization. A new approach has been recently drawn much interest in the industry to convert preformed silica or silica-alumina derived from the clays to zeolite Y. This novel procedure, which is closely related to the catalytic impregnation, presents a challenge to make zeolite Y. This new method was reported recently.4-8 However, the details of this approach and a discussion on the effects of the silica sources on the final product are poorly reported. In the impregnation technique, an active metal in the solution is added slowly to a porous catalyst support without any agitation.1 After complete adsorption of the active metal, the slurry undergoes drying and calcination. Even though using precipitated silica might be more expensive than the other silica sources, this innovative technique allows one to eliminate the agitation and the preparation of silica solution to form the gel. In this work, attempts were made to prepare the pure form of zeolite Y using this new approach. This was achieved by * To whom correspondence should be addressd. E-mail: srohani@ uwo.ca.
studying the influences of the silica sources on the crystal structure of the resulting zeolite Y. There are numerous studies related to the hydrothermal synthesis of zeolites investigating the effects of many factors.9,10 2. Experimental Section 2.1. Synthesis. The following materials were used: (1) Sylopol 948 (92% w/w SiO2, Grace, Baltimore, MD); (2) fumed silica (pure silica, particle size: 14 nm, Sigma-Aldrich, St. Louis, MO); (3) silica sol (40% w/w SiO2, Ludox HS40, SigmaAldrich, St. Louis, MO); (4) silica gel (Purasil 60A, Whatman, Florham Park, NJ); (5) soluble silicate (7.75% w/w Na2O, 27% w/w SiO2, 65.25% w/w H2O; Sigma-Aldrich, St. Louis, MO); (6) sodium aluminate (38.8% w/w Na2O, 54.2% w/w Al2O3, 8% w/w H2O; Strem Chemicals, Newburyport, MA); (7) aluminum sulfate hexadecahydrate and aluminum nitrate nonahydrate (minimum 98% pure, Sigma-Aldrich, St. Louis, MO); (8) sodium hydroxide (>99% w/w NaOH; Alphchem, Mississauga, ON); (9) hydrochloric acid (37% w/w Sigma-Aldrich, St. Louis, MO); and (10) distilled water. The zeolite Y was prepared according to the following procedure. In a 25 mL flask, 0.13-0.2 g of NaOH and 0.145 g of NaAlO2 were added to 1-2 mL of water with stirring until a clear solution was obtained (according to the equilibrium diagram for the Na2O-Al2O3H2O system at 30 °C,11 the solution prepared from 0.13 g of sodium hydroxide, 0.145 g of sodium aluminate, and 1 mL water (containing 6.7% Al2O3 and 12.2% Na2O) gives one point on the equilibrium line between the stable sodium aluminate and the supersaturated solution with respect to aluminum hydroxide). The resulting solution was added to 0.3 or 0.5 g of silica source (based on the pure SiO2) in a 30 mL Teflon bottle without stirring and left for 48-120 h for aging. The composition of the resulting synthesis gel was 3-4Na2O:1Al2O3:6 or 10SiO2:70-200H2O. The bottle was heated to 100 °C in an oven and kept there for 48 h. The bottle was then cooled to room temperature, and the product was suspended in water and filtered by vacuum. Finally, the product was dried at 100 °C for 2 h. The experiments conducted the same procedure for the various silica sources, and yielded products of different purity, different amounts of conversion to zeolite Y (depending on the
10.1021/ie801338u CCC: $40.75 2009 American Chemical Society Published on Web 04/20/2009
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Table 1. Physical Properties of the Used Silica Sources silica sources
BET specific area (m2/g)
particle sizes (µm)
Sylopol 948 precipitated silica PS-1 precipitated silica PS-2 fumed silica silica sol silica gel precipitated silica-alumina
304 165 252 200 220 550 160
40-100 20-60 30-80 0.014 0.012 38-63 10-30
amount of impure zeolite phases). However, excellent reproducibility of the experimental results for one silica source was achieved. The product yield (including both the crystalline and the amorphous phases) was calculated in terms of percentage of theoretical yield that was 0.5 g of pure zeolite Y, with a representative chemical composition of Na2O:Al2O3:4.8SiO2: 9H2O.3 The calculation was based on equal amounts of alumina in the gel composition and zeolite Y chemical formula. First, the required alumina amount was calculated by the simple stoichiometric relationship: alumina amount ) mz× (MAl/Mz) where mz and MAl and Mz are the weight of zeolite Y to be synthesized, the molecular weight of alumina, and the molecular weight of zeolite Y. The amount of alumina was assumed as the basis for calculating the amounts of the components in the chosen gel composition. After synthesis, the weight of the formed zeolite Y was determined (mz′). % yield was obtained from dividing two quantities: % yield ) mz′/mz × 100 The required amounts of the components mentioned above were calculated to yield 0.5 g of zeolite Y. In the case of low water content gel (H2O/Al2O3 mole ratios in the range of 35-50), due to the restriction of sodium aluminate solubility as described above, 0.61 g of aluminum nitrate nanohydrate was dissolved in 0.5 mL of water. The solution was added to 0.3 g of silica gel without stirring and was kept to complete dryness. The silica gel was calcined at 450 °C in an electrical furnace for 3 h. Next, a 0.5 mL solution containing 0.13 g of NaOH was added to silica-alumina gel without stirring. The rest of the experiment was conducted according to the same procedure described above. The gel composition was 2Na2O:Al2O3:6SiO2:35H2O. It should be noted that 0.5 mL of solution was enough to make the whole silica gel wet (like a paste) (0.3 g of silica). The precipitated silica and silica-alumina were prepared using the conventional procedures of the precipitation of the soluble silica by adding simultaneously sulfuric acid or sodium aluminate solutions under the controlled pH and temperature. For example, precipitated silica PS-1 was prepared by adding 5 mL of 10% SiO2 soluble silicate solution to 5 mL of 2 molar sulfuric acid solution simultaneously at the controlled flow rates to maintain the pH of mixture constant around 9 at room temperature. The precipitated silica was washed twice to remove all of the formed salts, filtered, and dried in an oven for 5 h at 100 °C. The dried products were used for zeolite Y synthesis as described above. 2.2. Characterization. Powder XRD patterns were recorded by X-ray diffractometer (Rigaku Miniflex, Houston, TX) using Cu KR radiation. The d-spacing and relative peak heights of the unknown samples were matched to reference crystal structure. The crystallinity and the phase purity of synthesized samples were also analyzed by powder X-ray diffraction
patterns. The degree of crystallinity (purity) was estimated roughly by comparing the intensity of the first highest peak appearing at 2θ ) 6.18° to that of the same peak in the powder XRD pattern of the pure sample (Strem sample purchased from Strem Chemicals, Newburyport, MA). The height of the pure sample was taken as 420 (arbitrary unit) as 100% peak height by Jade 7 XRD MDI library. The method used for the estimation of percent crystallinity was ASTM D3906-0312 for zeloite Y or D5357-03 for zeolite A. The method involves dividing the area of all or some of the peaks (SX) by that of the reference sample (SR). Therefore:12,13 percent crystallinity of zeloite Y or zeloite A ) SX/SR × 100 The BET surface areas (SBET) and pore size distributions were determined by nitrogen adsorption-desorption on an ASAP2010 instrument (Micromeritics Instrument Corporation, GA), and the pore size distributions were calculated by the advanced Barrett-Joyner-Halenda (BJH) method using the adsorptiondesorption branches of the isotherms. Prior to these measurements, the samples were degassed at 170 °C in a vacuum. The SEM pictures were taken by a Hitatchi S-4500 field emission SEM. The SiO2/Al2O3 ratio of synthesized zeolite Y was determined by an EDX (energy dispersive X-ray) analysis system attached to SEM. Solid-state 27Al NMR experiments were performed on a narrow-bore Varian Infinity Plus 600 spectrometer (νL(27Al) ) 156.3 MHz) using a triple-resonance Varian T3 MAS probe. The samples were ground and packed tightly into 3.2 mm o.d. ZrO2 rotors and rotated at 15.0 kHz. The 27Al NMR spectra were obtained using a one-pulse experiment using a 3.5 µs 27Al π/2-pulse, a 10 s pulse delay, and a 100 kHz sweep width and were referenced to a 1.0 M solution of aluminum chloride (δ(27Al) ) 0.0 ppm). A total of 120 transients were collected for the Strem sample, while 800 transients were collected for the synthesized sample. The FIDs were anodized with 20 Hz line-broadened and zero-filled two times before Fourier transform. 3. Results and Discussion Over the course of this investigation, seven different silica sources were tried. These were: (1) Sylopol 948, (2) fumed silica, (3) silica sol, (4) Purasil 60A silica gel, (5 and 6) the precipitated silica PS-1 and PS-2 prepared at the different precipitation conditions, and (7) the precipitated silica-alumina prepared by coprecipitating the soluble silicate solution with aluminum salt or sodium aluminate solutions. Table 1 lists the two important physical characteristics of these silica sources, the BET surface area and the particle size. Throughout this study, the molar ratio of the reactants used was: 3-4Na2O: Al2O3:6 or 10SiO2:70-200H2O. All samples were prepared without stirring, aged for 48-120 h at room temperature, and crystallized for 48 h at 100 °C in an oven. The results of powder XRD and BET analyses are summarized in Table 2 along with the pure Strem sample. As shown in Table 2, the properties of the zeolite prepared using Sylopol 948 are similar to those of the standard zeolite (pure Strem sample). The yield of the purest product was calculated as 65% theoretical yield as described in section 2.1. For the other samples, the theoretical yield varied in the range of 50-60% depending on the extent of the product purity. The results of the two last silica sources are not presented in Table 2, because using silica gel (Purasil 60A) and coprecipitated silica-alumina did not yield pure zeolite Y. In the latter two cases, the impure zeolite Y was primarily amorphous aluminosilicate. The same results were
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Table 2. Results of Powder XRD and BET Analyses for Different Samples silica sources
Strem NaY
Sylopol 948-1
Sylopol 948-2a
precipitated silica PS-1
precipitated silica PS-2
fumed silica
silica sol
Sylopol 948 LW
% yield % crystallinity SiO2/Al2O3 mole ratio intensity of first peak extra peaks BET specific area m2/g micropore area m2/g micropore PV cm3/g pore volume cm3/g
100 4.7 420 no 731.5 687 0.32 0.356
65 100 4.2 496 no 734.5 688 0.321 0.357
63.2 100 4.2 444 no 710 667.6 0.31 0.34
57 89 3.7 369 yes 615 573 0.267 0.302
63.5 100 4.35 457 no 683 640 0.3 0.33
55 85 3.5 270 yes 585 547.4 0.255 0.28
60 96 3.82 323 no 705 661 0.308 0.35
52.5 72.6 3.4 210 no 681 637 0.3 0.35
a
Sample aged for 48 h.
Figure 1. Typical XRD patterns of synthesized zeolite Y (using Sylopol 948 as a silica source) (b) and Strem pure zeolite Y (a).
Figure 2. SEM image of Strem zeolite Y particle at 600 nm spatial resolution.
Figure 3. SEM image of the synthesized zeolite Y (using Sylopol 948 as a silica source) particle at 600 nm spatial resolution.
obtained for zeolite Y syntheses at the same conditions using the silica gel prepared by gelling the soluble silicate by adding the mineral acid solution under vigorous agitation as well as
cogelling of the soluble silicate by sodium aluminate solution. The typical powder XRD pattern of synthesized zeolite Y (using Sylopol 948 as a silica source) is shown in Figure 1b along
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Figure 4. Typical BJH pore size distribution for Strem zeolite Y.
Figure 5. Typical BJH pore size distribution for synthesized zeolite Y (using Sylopol 948 as a silica source).
with the powder XRD of the standard Strem pure zeolite Y (Figure 1a). The SEM images of the particles of the synthesized zeolite Y and Strem zeolite Y are shown in Figures 2 and 3 at 600 nm spatial resolution. From the SEM image, it can be seen
that the synthesized sample is crystalline and that crystals are mostly cubic. SiO2/Al2O3 mole ratios of the synthesized zeolites Y and Strem zeolite Y measured by EDX were presented in Table 2. Figures 4 and 5 show the typical BJH pore size
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Figure 6. Isothermal plot for Strem zeolite Y.
Figure 7. Isothermal plot for the synthesized zeolite Y (using Sylopol 948 as a silica source).
distributions for Strem zeolite Y and our prepared pure product using Sylopol as a silica source. As seen in these plots, there is close similarity between the Strem pure sample and our pure product. These graphs also prove that our samples are mi-
croporous, due to the very sharp semipeak around 2 nm pore diameter. Nitrogen isotherms for the Strem sample and the synthesized sample are plotted in Figures 6 and 7. These plots show exactly the same trends for both samples. Figure 8 shows
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Figure 8. Solid-state 27Al NMR spectra for Strem zeolite Y as a reference and the synthesized zeolite Y (using Sylopol 948 as a silica source).
the solid-state 27Al NMR spectra for the two above-mentioned samples. High-resolution solid-state 27A1 NMR spectra of assynthesized zeolites generally contain a single signal corresponding to tetrahedrally coordinated Al (see Figure 8). The apparent chemical shift of this signal measured in a 9.4 T magnetic field ranges, for different materials, from about 51 to 65 ppm from Al(H20)6.3,14 The experimental observations suggest that the purity of the synthesized zeolite Y depends strongly on the BET specific surface areas of the silica sources. As presented in Table 2, the sequence of the silica sources giving the purest zeolite Y is precipitated silica, silica sol, and fumed silica. The powder precipitated silica (Sylopol 948 and prepared PS-2) was an excellent silica source to prepare the lower water content and low alkalinity gel. The gel composition achieved was 2Na2O: Al2O3:6SiO2:35-50H2O. An alkalinity lower than 2 did not
result in zeolite Y formation. The minimum water content in the gel compositions was 35 mol, which was enough for covering the precipitated silica entirely and turning it into a dense paste. The result is shown in the last column of Table 2 (sample Sylopol 948 LW). The precipitated silica impregnated with sodium aluminate was allowed to dry completely, and then sodium-containing solution was added. The rest of the experiment was conducted similar to the procedure described above. There was no change in the result. Even the calcination of the impregnated precipitated silica at 600 °C for 5 h, to decompose the adsorbed sodium aluminate on the precipitated silica into alumina, did not change the result obtained. In the case of high water content gel, it was not necessary to carry out the above process as it was done for the low water content gel, due to sodium aluminate solubility limitation at room temperature. The visual observation of the
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impregnated precipitated silica, during aging, showed that the precipitated silica kept its porous structure. During heating, the precipitated silica maintained its backbone structure for 20 h. This was followed by a gradual transformation to zeolite Y, during which the synthesizing mixture became colloidal and led to the solid-liquid separation. The important observation was related to the percent purity of the product zeolite Y. In general, as is shown in Table 2, the percent yield decreased as the product purity decreased. The precipitated silica resulted in very pure product with high percent yield (see Table 2), while the silica gel resulted in impure zeolite Y and low percent yield. This may be explained by the difference in the BET specific surface areas. Note that the precipitated silicas have a surface area ranging from 150 to 300 m2/g (the same as the surface areas of catalyst supports, such as γ-alumina), and silica gel has a surface area over 500 m2/g. Particle size distribution did not have a significant effect on the purity of the resulting zeolite. The other catalyst preparation technique, coprecipitation, did not result in the pure zeolite Y. In this procedure, alumina solution obtained from dissolving aluminum sulfate or nitrate along with the silica solution (silica sol or soluble silicate) were simultaneously added to a specified amount of water in a beaker. The precipitated silica-alumina was filtered, washed to remove the salts formed, and dried. The powder was used as a silica source. Conducting similar experiments under the same conditions did not result in zeolite Y. The product was amorphous aluminosilicates. These experiments revealed that the precipitated silica-alumina did not show the same reactivity to make zeolite Y as the precipitated silica impregnated with alumina. We believe that the difference in two techniques can be attributed to the solid-phase transformation mechanism in which the amorphous gel is thought to reorganize to form crystals. The data obtained using the complementary techniques confirmed that after mixing of the initial reactants, sodium progressively diffuses to the bulk sodium-poor gel particles, breaking the gel structure and reorganizing the aluminosilicate species.15,16 Later, mass transformation of the amorphous phase into zeolite Y occurs when the chemical composition of the amorphous gel approaches the stoichiometric SiO2/Al2O3 mole ratio of zeolite Y. The formation of the large-size particles and the change in the SiO2/Al2O3 ratio during crystallization indicate that there is aluminum distribution inside the particles. Therefore, aluminum distributes inside the particles, and the formation of zeolite Y (high-silica faujasite) is enhanced by the initially formed zeolite X (Al-rich faujasite).17,18 In this regard, the precipitated silica (for example, Sylopol provides an excellent support for the polymerization catalysts and PS-2) has well-defined pores. Inside these pores, aluminum element is distributed uniformly to form the active sites and induces the nucleation of faujasite structure. Thus, the uniform distribution of alumina and its loose bonding to the exposed surface of support (silica l) contribute to the easy formation and dispersion of the nucleation sites and their interaction with the sodium ions diffusing into particles from solution. Using the precipitated silica impregnated with alumina (in low alkalinity, low water content gel composition) yielded zeolite Y that supports this supposition. 4. Conclusions The pure form of zeolite Y was prepared by a novel process. The proposed process is similar to the catalytic impregnation process and uses powder silica gel. This new approach allows one to eliminate the need for vigorous agitation and the preparation of silica solution in the mixing stage. The whole process can be conducted under static conditions in one single vessel (except for the preparation of alumina-containing solution). It was found that the BET specific surface area of the
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silica sources was the most important factor affecting the purity of the zeolite. The coprecipitation method did not result in any zeolite Y under our selected synthesis conditions. A heterogeneous mechanism is proposed in the synthesis of zeolite Y and the essential role that the reactive aluminosilicate precursor plays in the crystallization stage. The results of this study showed that the formation and uniform distribution of the desirable aluminosilicate precursors inside the pores and on the surface of the silica support resulted in the conversion of the amorphous gel to zeolite Y. First, aluminum precursor in solution is adsorbed on the porous precipitated silica support, and then the adsorbed precursor reacts with the diffusing sodium to generate the active sites similar to the nucleating centers proposed in the conventional method of zeolite Y synthesis. Finally, these active sites transform the amorphous precipitated silica to faujasite by rearranging aluminosilicate structure. Therefore, the surface area of silica source influences the formation and stability of the active sites. The particle size and the geometric shape of silica sources did not affect significantly the purity of the product. Literature Cited (1) Ertl, G.; Knz¨inger, H.; Weitkamp, J. Handbook of Heterogeneous Catalysis; Wiley-VCH: Weinheim, 1997; Vol. 4. (2) Barrer, R. M. Hydrothermal Chemistry of Zeolites; Academic Press: London, 1982. (3) Breck, D. W. Zeolite Molecular SieVes: Structure, Chemistry and Use; Krieger: Florida, 1984. (4) Lam, Y. L.; Saavedra, A.; Costa, A. F.; Da Silva Santos, A.; Moure, G. T.; Roncolatto, R. E.; Stamires, D.; O’Connor, P. Molecular sieves of faujasite structure. U.S. Patent 6,756,029, 2004. (5) Murrell, L. L.; Overbeek, R. A.; Chang, Y. F.; Van der Puil, N.; Yeh, C. Y. Method for making molecular sieves and novel molecular sieve compositions. U.S. Patent 6,004,527, 1999. (6) Koegler, J. H.; Yeh, C. Y.; Angevine, P. J. Nanocrystalline inorganic based zeolite and method for making same. U.S. Patent 6,793,911, 2004. (7) Mu¨ller, U.; Ma, L.; Feng-Shou, X.; Yang, X. Process for preparing a nanosized zeolitic material. U.S. Patent 7,211,239, 2007. (8) Zhou, J.; Min, E.; Yang, H.; Zong, B. Y-zeolite-containing composite material and a process for preparing the same. U.S. Patent 7,067,449, 2006. (9) Yang, S.; Vlessidis, A. G.; Evmiridis, N. P. Influence of Gel Composition and Crystallization Conditions on the Conventional Synthesis of Zeolites. Ind. Eng. Chem. Res. 1997, 36, 1622. (10) Koroglu, H. J.; Sarioglan, A.; Tatlier, M.; Senatalar, A.; Savasci, T. Effects of Low-Temperature Gel Aging on the Synthesis of Zeolite Y at Different Alkalinities. J. Cryst. Growth 2002, 24, 1248. (11) Keller, R. J.; Len, J. J. Aluminates. In Kirk-Othmer Encyclopedia of Chemical Technology; Othmer, D. F., Mark, H. F., Grayson, M., Eckroth, D., Eds.; John Wiley and Sons: New York, 2003; pp 273-279. (12) ASTM International Standards Worldwide Home Page. http://www. astm.org (accessed May 1996). (13) Cundy, C. S.; Cox, P. A. The Hydrothermal Synthesis of Zeolites: Precursors, Intermediates and Reaction Mechanism. Microporous Mesoporous Mater. 2005, 85, 1. (14) Engelhardt, G.; Michel, D. High Resolution Solid-State NMR of Silicates and Zeolites; John Wiley and Sons: New York, 1987. (15) Valtchev, V.; Rigolet, S.; Bozhilov, K. N. Gel Evolution in a FAUtype Zeolite Yielding System at 90 °C. Microporous Mesoporous Mater. 2007, 101, 73. (16) Ueda, S.; Kageyama, N.; Koizumi, M.; Kobayashi, S.; Fujiwara, Y.; Kyogoku, Y. Role of Soluble Species in the Crystallization of Mordenites. J. Phys. Chem. 1984, 88, 2128. (17) Ogura, M.; Kawazu, Y.; Takahashi, H.; Okubo, T. Aluminosilicate Species in the Hydrogel Phase Formed during the Aging Process for the Crystallization of FAU Zeolite. Chem. Mater. 2003, 15, 2661. (18) Thangaraj, A.; Kumar, R. NMR Study of Soluble Aluminosilicate Precursors in Zeolite Y Synthesis. Zeolites 1990, 10, 117.
ReceiVed for reView May 9, 2008 ReVised manuscript receiVed February 12, 2009 Accepted February 12, 2009 IE801338U
