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Ind. Eng. Chem. Res. 2000, 39, 4944-4948
Sol-Gel Synthesis of Silicalite/γ-Alumina Granules Zhaohui Yang and Y. S. Lin* Department of Chemical Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0171
Spherical silicalite/γ-Al2O3 composite granules of about 2 mm in diameter were prepared by a sol-gel process from a mixture of the stable boehmite and silicalite sols. The stable silicalite sol was prepared by a hydrothermal method. Silicalite/γ-Al2O3 composite granules with a zeolite content of up to 70 wt % were formed by the sol-gel “oil-drop” process in which a paraffin oil and ammonia solution were used as the media to facilitate shape forming and consolidation of the gel structure. The sol-gel-derived silicalite/γ-Al2O3 granules have a pore structure reassembling that of a mixture of silicalite and γ-Al2O3 powders. The crush strength of the sol-gelderived silicalite granules is about six times that of a similar commercial silicalite granule. In addition, the sol-gel-derived silicalite/γ-Al2O3 granules exhibit better sorption properties for SO2 at a lower SO2 partial pressure than pure silicalite powder. 1. Introduction With the increasing concerns about energy, the adsorption process has become more competitive in many industrial processes.1,2 It is well-known that the performance of the adsorption process is highly dependent on the properties of the adsorbents, in which large surface area, uniform particle size, narrow pore size distribution, good chemical and thermal stability, and high mechanical strength are required. Unfortunately, it is difficult to prepare adsorbents by the conventional methods that meet all of the requirements, and the improvement of one property usually comes at the expense of the others. Zeolites have been used extensively in adsorption process because of their “molecular sieving” effect, their ability to selectively take up some molecules into their porous structure while rejecting others on the basis of their larger effective molecular dimensions. Generally, industrial zeolite adsorbents have the shapes of cylinders, extrudes, and spheres.3 Preparation of commercial zeolite particles still relies on the conventional methods, such as “rolling” and “extrusion”.4-7 The zeolite particles derived from these processes suffer from poor mechanical strength and short lifetimes, making them problematic for applications in moving bed or fluidized bed processes. Despite its high cost in the preparation of materials, the sol-gel process has attracted increasing attention in recent times with the increasing demand for advanced ceramics with high purity, homogeneity, and well-controlled properties.9 One major advantage of this technology compared to other conventional preparation methods is the ability to proceed from molecular precursor to final products, enabling the intimate control over all stages of processing.8,9 Balducci et al.10 recently patented a process for the preparation of silica/zeolite composite spheres in which the composite material contains up to 70 wt % of titanium silicalite, beta zeolite, or mixtures of titanium silicalite/beta zeolite. The average diameters of the spheres was in the range of 20 to 150 µm. The prepared particles are characterized by high mechanical resistance and a high surface area of * Corresponding author. Tel.: +1 (513) 5562761. Fax: +1 (513) 5563473. E-mail:
[email protected].
between 300 and 800 m2/g. However, the particles prepared are in small size, making them unsuitable for industrial applications. Recently, Lin and co-workers11-13 reported sol-gel synthesis of spherical pure γ-Al2O3 granules of about 2-5 mm in diameter by using a method that combines the Yoldas process and an “oil-drop” granulation method. The sol-gel-derived γ-Al2O3 granules exhibit a much higher crush strength and attrition resistance than γ-Al2O3 granules prepared by the conventional processes.11-13 In the present work, this sol-gel method was extended to prepare spherical zeolite (silicalite) granules. The main objective was to prepare zeolite granules with enhanced mechanical strength and improved uniformity, which are critical to their applications in moving and fluidized bed processes. The SO2 sorption properties of the new silicalite/alumina granules were also studied in this work because one of the potential applications of these granules is for flue gas desulfurization. The present paper reports the synthesis and properties of the sol-gel-derived zeolite/alumina granules. 2. Experimental Section 2.1. Synthesis. Silicalite sol was synthesized according to the following procedure: (1) NaOH (0.75 g, 99%, Aldrich) was dissolved in 50 mL of a 1 M tetrapropylammonium hydroxide (TPAOH) solution in a capped Teflon flask at 80 °C with vigorous stirring. (2) Fumed silica fine powder (10 g) was added to the aforementioned solution with stirring until a clear solution was obtained. (3) The clear solution was cooled to room temperature, aged for 3 h, and then hydrothermally treated in a Teflon-lined stainless steel autoclave at 120 °C for 12 h. (4) The obtained suspension was centrifuged at 14 000 rpm and washed with deionized water to adjust the pH of the suspension to the range of 9.510.0. Boehmite sol (2 M) was prepared by the Yoldas process, and the precursor used in the preparation was aluminum tri-sec-butoxide. The details were given elsewhere.13 Mixed sols were obtained by adding dropwise the asprepared silicalite sol (15 wt %) into the 2 M boehmite sol with strong stirring. During the mixing process, 1 N HNO3 was added to the sol mixture. The amount of
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HNO3 added was regulated so that the pH of the sol mixture was maintained in the range of 3.0-3.5. The sol mixture was then heated slowly to 75 °C until the mixture became so viscous that it could no longer be stirred by the stirrer (Thermolyne Sybron Type 1000) at the maximum power. This partially gelled sol mixture was then used to prepare the granules by the oil-drop method.13 During the granulation process, the partially gelled silicalite/boehmite hybrid sol was transferred to droppers for drop formation. The sol droplets fell through a paraffin oil layer. Spherical wet gel particles were formed because of the surface tension. The wet-gel particles then fell into an ammonia solution placed right underneath the paraffin oil. After aging in the ammonia solution for about 1 h, the soft wet-gel particles became rigid. These particles were then withdrawn from the ammonia solution, carefully sieved, and washed with water and acetone to eliminate the oil and ammonia residues. The drying process was completed in an oven at 40 °C and high humidity for 24 h. The dry granules were then calcined at 450 °C for 20 h with a heating and cooling rate of (40 °C/h. The template (TPAOH) in the silicalite was removed in this calcination step. 2.2. Characterization. The appearance, shape, and size of the granules prepared above were examined by light microscopy (Olympus, type SZH). The crush strength of the individual particles was determined by a universal testing machine (Instron 4465), and the maximum force applied to break the single particle was taken as the crush strength. XRD analysis (Siemens D-50, Cu KR1 radiation) was conducted to test the crystalline structure of the silicalite/γ-Al2O3 composite granules. The pore structure of the particles was studied by nitrogen adsorption porosimetry with a micropore feature (Micromeritics, ASAP 2000). The BET surface area was calculated from the adsorption isotherm by applying the BET adsorption model. The pore volume was determined as the liquid equivalent of the volume adsorbed at the relative pressure closest to 1 on the adsorption isotherm, and the average pore size was estimated from the ratio of the pore volume to the BET surface area as 4V/A. The micropore volume was calculated from the intercept on the Y axis of the t-plot from adsorption isotherm, and the micropore surface area was obtained by subtracting the external surface area from the BET surface area, in which the external surface area was calculated from the product of the slope of the t-plot and the density conversion factor. All of the calculations on pore structure were done using the software provided by Micromeritics. The SO2 adsorption on these granules was studied by a fixed bed. The fixed bed included a gas delivery system, an adsorber column, a UV detector, and a computer data acquisition system. A dense ceramic tube of 6 mm i.d. and 9 mm o.d. loaded with 1.5 g of fresh silicalite granules was used as the adsorber. The SO2 concentration of the effluence was measured by a Varian spectrophotometer (Cary 50) at a wavelength of 285 nm. Adsorption experiments at different temperatures and SO2 concentrations were performed, and the adsorption capacities were calculated from the breakthrough curves. Two cylinders of dry air containing 1971 and 2993 ppm of sulfur dioxide were used as the source of SO2, and lower SO2 partial pressures were obtained by mixing the SO2 from the above cylinders with the dry air. To
Table 1. Parameters of Fixed-bed Adsorber parameter
value
adsorbent packed bed length bed porosity SO2 feed concentration temperature flow rate
1.5 g 6 cm 0.48 986, 1971, 2993 ppm 22, 50, 100 °C 60 mL/min
Table 2. Optimum Conditions for Preparation of Silicalite/γ-Al2O3 Composite Granules parameters
optimum results
pH of silicalite sol pH of the boehmite sol concentration of the boehmite sol concentration of silicalite sol highest silicalite content ammonia concentration gelation temperature gelation time aging time height of the paraffin layer rotation speed of the motor radius of the wet sphere
9.5-10.0 3.6 1.5-2.0 M 15.0-20.0 wt % 70 wt % 10 wt% 75 °C 1.0-1.5 h 1.0 h 25 cm 20 rpm 2.0-3.0 mm
study the reversibility of the adsorption process, desorption was performed by sweeping through the adsorber with dry air at a higher temperature, i.e., 400 °C. The experimental conditions of the fixed-bed adsorption are listed in Table 1. 3. Results and Discussion 3.1. Preparation of Silicalite/γ-Al2O3 Granules. Under the conditions of hydrothermal synthesis mentioned above, the average particle size of silicalite in the silicalite sol, observed by SEM, is around 100 nm. At the pH range of 9.5-10, the silicalite suspension exhibits excellent stability at room temperature, and no noticeable precipitate was observed after the sol was stored in a capped flask at room temperature for 2 months. The solid concentrations of the as-prepared silicalite and boehmite sols are 21.2 and 13.8 wt %, respectively. The operating parameters of the granulation process are critical for the properties of the final product. Table 2 lists the optimal conditions for the process. The pH, aging temperature, and time for the sol mixture prior to drop formation should be carefully controlled. In this work, the pH of the sol mixture was adjusted to the range of 3.0-3.5 by addition of an appropriate amount of HNO3, depending on the solid concentration of the sol mixture and the silicalite content. The sol mixture was aged at 75 °C for 1-1.5 h. The paraffin solution was 25 cm in height, which provided enough time for the sol drops to form spherical particles. The wet spherical granules stayed in the ammonia solution, at an optimum concentration of 10 wt %, for 1 h to consolidate the structure of the gel particles. Under these conditions, the granulation process was easy to control, and the wet granules were sufficiently strong to retain good integrity following the washing step. After being dried, the granular particles shrank 50-70% in size and increased markedly in mechanical strength. In this study, silicalite/γ-Al2O3 granules containing 40, 50, and 70 wt % silicalite were prepared under the listed conditions. It was found that, with the increase of the silicalite content, especially when it is higher than 70 wt %, the formation of wet spheres in the ammonia solution becomes more difficult. At 70 wt % or lower
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Figure 1. Optical picture of granular particles.
Figure 2. XRD patterns of (A) pure silicalite and 50% silicalite/ γ-Al2O3 granules (B) before and (C) after sintering.
silicalite content, the prepared granules are of good quality. Figure 1 shows an optical micrograph of the granular particles containing 50 wt % of silicalite. From the picture, nice sphericity and uniform size can be observed. The as-prepared particles have a size of around 2 mm in diameter, which is suitable for use as an adsorbent. 3.2. Granule Characteristics. Figure 2 shows the XRD patterns of the pure silicalite and 50 wt % silicalite/ γ-Al2O3 composite granules before and after calcination. Comparing the measured XRD patterns with the standard XRD pattern of silicalite (MFI)14 shows that the silicalite powder prepared by the hydrothermal synthesis has the crystal structure of an MFI-type zeolite. Both silicalite/γ-Al2O3 composite granules exhibit the main XRD peaks of silicalite crystal structure, as shown in Figure 2B and C. The 2θ values of the peak positions of both silicalite/γ-Al2O3 composite materials are almost identical, and the intensity of the calcined sample is slightly larger than that of the sample before calcination. The nitrogen adsorption and desorption isotherms at liquid nitrogen temperature for granules with different silicalite contents are shown in Figure 3. The isotherm for the pure γ-alumina granule clearly shows a hysteresis loop, characteristic of mesoporous materials. The hysteresis loop disappears, and the adsorption amount at low relative pressure increases with increasing silicalite content, which is also characteristic of micropores. The BET surface area, total pore volume, and average pore size and the micropore surface area and volume, calculated by the methods described in section 2.2, are summarized in Table 3.
As seen from Table 3, the BET surface area increases, and the average pore diameter and pore volume decrease, with increasing silicalite content. Pure silicalite has a BET surface area of 417.8 m2/g,15 much higher than that of γ-Al2O3. The pore size and pore volume of silicalite are, respectively, about 0.6 nm and 0.19 cm3/ g, much smaller than those of γ-Al2O3. It is expected that these values that quantify the microstructure of the silicalite/γ-Al2O3 composite granules should lie between those of the pure silicalite and pure γ-Al2O3. The experimental results are consistent with this expectation. Figure 4 shows a plot of the micropore surface area and volume of the composite granules versus the silicalite content. These data can be correlated by two straight lines going through the original. Because only silicalite will contribute to the micropore measured by nitrogen adsorption porosimetry, this linear dependency of the micropore structure data on the silicalite content indicates that the composite granules resemble a mixture of silicalite and alumina powder and that the presence of the alumina does not affect the microstructure of silicalite. The crush strengths of the three silicalite/γ-Al2O3 composite granules with different silicalite contents are given in Table 3. The crush strength of the granules with 40 and 50 wt % silicalite is as good as that of solgel-derived pure γ-Al2O3 granules. The granules with 70 wt % silicalite have a slightly lower crush strength (93 N/particle). The crush strength of a similar commercial silicalite granule (containing 20-30 wt % alumina binder) (S-115 from UOP) is only 16 N/particle measured under the same conditions. The crush strength of the sol-gel-derived zeolite/alumina composite granules is about 6 times that of the commercial silicalite granule. The structure of the obtained granular particles is mainly determined by the mechanism of the granulation process. It is possible that, at 70% or lower silicalite content, γ-Al2O3 is present as a continuous phase and silicalite is the dispersed phase in the granular particle. During the sol-gel process, a continuous network of AlOOH was formed, and silicalite crystallites were imbedded in the network. Calcination at 450 °C resulted in the transformation of AlOOH to γ-Al2O3 crystallites, which were bound together in the continuous phase through partial sintering or coarsening. Silicalite crystallites could be retained in the alumina network by physical entrapment or partial sintering among silicalite crystallites or between silicalite and alumina crystallites. The strong mechanical strength is provided by the strong binding between the nanoparticles of γ-Al2O3. At higher silicalite content, alumina might no longer be present as a continuous phase. This could be the reason for the extreme difficulty in preparing the silicalite/ alumina granules with good integrity at higher silicalite contents. 3.3. SO2 Adsorption Properties. The adsorption of SO2 on 50 wt % silicalite/γ-Al2O3 granules was studied. The SO2 breakthrough curves at different feed SO2 concentrations and temperatures are given in Figures 5 and 6, respectively. The temperature swing on the fixed bed showed that the silicalite/alumina composite granules could be fully regenerated at higher temperatures. From the breakthrough curve, the adsorption amounts can be determined by integrating the area bound by the breakthrough curve, the C/C0 axis, and the straight horizontal line at C/C0 ) 1. For example,
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Figure 3. N2 adsorption isotherms of four materials: (A) pure γ-Al2O3, (B) 40% silicalite/γ-Al2O3, (C) 50% silicalite/γ-Al2O3, and (D) 70% silicalite/γ-Al2O3.
Figure 4. Microporous surface area and volume of the sililcalite/ γ-alumina granule versus silicalite content (wt %).
Figure 5. Breakthrough curves at different feed concentration.
Table 3. Properties of Silicalite/γ-Al2O3 Composite Granules 40% 50% 70% γ-Al2O3 silicalite silicalite silicalite BET surface area (m2/g) pore volume (cc/g) avg pore diameter (Å) micropore area (m2/g) micropore volume (cc/g) crush strength (N/particle)
284 0.447 63.1 0 0 104
327 0.406 49.6 136 0.064 104
350 0.389 44.5 188 0.089 109
393 0.375 38.2 255 0.12 93
at 22 °C, the adsorption amounts of SO2 are 0.208, 0.305, and 0.386 mmol/g at PSO2 of 986, 1971, and 2993 ppm, respectively. The adsorption amounts of the silicalite/γ-Al2O3 granules at different temperatures are listed in Table 4. The Langmuir adsorption isotherm equation (see Table 4) was used to fit the equilibrium data shown in Table 4. Table 4 lists the regressed parameters qs and b at different temperatures. The regressed isotherm data are compared with the experimental data in Figure 7. The equation fits the experimental data well. The sorption equilibrium constant, Ke, defined as the slope of the adsorption isotherm at zero SO2 pressure,
Figure 6. Breakthrough curves at different temperatures.
can be calculated from the product of constants qs and b. Values of Ke for adsorption of SO2 on the sol-gelderived 50% silicalite/γ-Al2O3 are compared in Table 5 with those on the pure silicalite powder (S-115 from UOP).15 The heat of adsorption, ∆H, is calculated from the Ke values at different temperatures using the van’t Hoff equation. As shown, the sol-gel-derived composite granules have higher sorption equilibrium constants with a similar heat of adsorption compared with those
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Table 4. Adsorption Capacity and Langmuir Model Parametersa adsorption amount (mmol/g)
model parameters
temperature (°C)
986 ppm
1971 ppm
2993 ppm
qs (mmol/g)
b (kPa-1)
25 50 100
0.2025 0.1276 0.0480
0.3052 0.1971 0.0863
0.3860 0.2459 0.1052
0.683 0.4459 0.2894
4.269 4.061 2.031
a
Langmuir equation: q ) qsbP/(1 + bP).
streams. Despite these advantages, the sol-gel-derived silicalite/alumina granules are more expensive than the commercial silicalite or pure γ-alumina granules mainly because of the expensive aluminum precursor used in this work. It is certain that use of a commercial boehmite powder as the aluminum precursor13 and scaling-up of the sol-gel granulation process16 will lead to a substantial reduction in the costs of the silicalite/ alumina granules. Acknowledgment We acknowledge the support of Zeochem Inc. on the work reported in this paper. Literature Cited
Figure 7. SO2 adsorption isotherm on silicalite/γ-Al2O3. Table 5. SO2 Sorption Equilibrium Constants Ke (mmol g-1 kPa-1) and Heat of Adsorption on Composite 50 wt % Silicalite/γ-Al2O3 Granule and Pure Silicalite Powder Ke at 25 °C Ke at 50 °C Ke at 100 °C ∆H (kJ/mol)
granule
powder
2.916 1.811 0.588 20.0
1.013 0.564 0.183 21.3
of the pure silicalite powder. This suggests that, in the sol-gel-derived composite silicalite granules, γ-Al2O3 not only acts as binder but also provides active sites for SO2 adsorption. The newly developed material has better sorption properties than the powder for removal of trace SO2 from contaminated gas streams. 4. Conclusions The sol-gel oil-drop granulation method was successfully extended to the preparation of silicalite/γ-Al2O3 composite granules from a mixture of the boehmite sol and a stable silicalite sol prepared by a hydrothermal synthesis method. By careful control of the granulation process, composite granules of about 2 mm in diameter containing up to 70 wt % silicalite were prepared. The sol-gel-derived silicalite granules have a crush strength about 6 times that of similar silicalite granules available commercially. Compared with pure silicalite powder, the sol-gel-derived composite silicalite granules have better SO2 sorption properties and offer potential for application as the sorbent for removal of trace SO2 from waste
(1) Ruthven, D. M. Principles of Adsorption and Adsorption Processes; John Wiley & Sons: New York, 1984. (2) Yang, R. T. Gas Separation by Adsorption Processes; Imperial College Press: London, 1997. (3) Cahen, R. M.; Ander J. M.; Debus, H. R. Preparation of Catalysts II; Elsevier: Amsterdam, 1979. (4) Sherrington, P. J.; Oliver, R. Granulation; Heyden & Son Ltd: London, 1981. (5) Plee, D. Zeolite granules with zeolitic binder. U.S. Patent 5,132,260, 1991. (6) Sulaymon, A. H.; Mahdi A. S. Spherical Zeolite-Binder Agglomerates. Chem. Eng. Res. Des. 1999, 77, 342. (7) Seidel, B.; Staudte, B. The Influence of Clay Binder Material on the Physical Properties of the CaNaA Molecular Sieve Used in a Hydrocarbon Separation Process. Zeolites 1993, 13, 92. (8) Klein, L. C.; Pope, E. J. A.; Sakka, S.; Woolfrey, J. L. SolGel Processing of Advanced Materials; American Ceramic Society: Westerville, OH, 1998. (9) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: San Diego, CA, 1990. (10) Balducci, L.; Ungarelli, R.; Tonni, C. Silica/zeolite composite materials in spherical form and process for their preparation. U.S. Patent 5,965,476, 1997. (11) Deng, S. G.; Lin, Y. S. Granulation of Sol-Gel-Derived Nanostructured Alumina. AICHE J. 1997, 43, 505. (12) Wang, Z. M.; Lin, Y. S. Sol-Gel Synthesis of Pure and Copper Oxide Coated Mesoporous Alumina Granular Particles. J. Catal. 1998, 174, 43. (13) Buelna, G.; Lin, Y. S. Sol-Gel-Derived Mesoporous GammaAlumina Granules. Micropor. Mesopor. Mater. 1999, 30, 359. (14) Treacy, M. M. J.; Higgins, J. B.; von Balloons, R. Collection of Simulated XRD Powder Patterns for Zeolites: Powder Pattern Identification Tables. Zeolites 1996, 16, 323. (15) Deng, S. G.; Lin, Y. S. Sulfur Dioxide Sorption Properties and Thermal Stability of Hydrophobic Zeolites. Ind. Eng. Chem. Res. 1995, 34, 4063-4070. (16) Buelna, G.; Lin, Y. S. Preparation of spherical alumina and copper-oxide-coated alumina sorbents by improved sol-gel granulation process. Microporous Mesoporous Mater., in press.
Received for review June 7, 2000 Revised manuscript received September 14, 2000 Accepted September 20, 2000 IE000562R
